Meldrum’s Acid By Hamish McNab DEPARTMENT OF CHEMISTRY, UNIVERSITY OF EDINBURGH, WEST MAINS ROAD, EDINBURGH EH9 355 1 Introduction In 1908, A. N. Meldrum reported that the condensation of malonic acid with acetone, in acetic anhydride containing a small amount of sulphuric acid, gave a white crystalline solid, C6H~04, which titrated as a monobasic acid and lost carbon dioxide on heating.l He reasonably stated that ‘there can hardly be a doubt’ that the compound was the carboxylic acid (1). Sadly, this structure proved to be quite wrong, though forty y:ars were to elapse before it was deduced that the condensation must involve only the carboxyl groups of the malonic acid. On this basis, Davidson and Bernhard2 correctly assigned the structure of ‘Meldrum’s acid’ as 2,2-dimethyl-1,3-dioxan-4,6-dione(2), whose properties therefore relate to those of other cyclic 1,3-diones such as dimedone (3) and barbituric acid (4).0 (1) (2) (3) (4) 2 Preparation The original method is invariably used for the preparation of 1,3-dioxan-4,6 diones2 (Scheme 1). The procedure has some generality, and derivatives have HO,CXCO,H + RlYR3 AcZO x WO,R R‘ 0 o+ R R‘Scheme 1 been made by condensing aliphatic3 or aromatic4 ketones, or aryl aldehyde^,^ A. N. Meldrum, J. Chem. SOC.,1908, 93, 598. a D. Davidson and S. A. Bernhard, J. Amer. Chem. SOC.,1948, 70, 3426. B. Eistert and F. Geiss, Chem. Ber., 1961, 94, 929. J. Swoboda, J. Derkosch, and F. Wessely, Monatsh., 1960, 91, 188. A. Michael and N. Weiner, J.Amer. Chem. SOC.,1936, 58, 680. Meldrum’s Acid with un-, mon0-,3?~ malonic acids. The reaction is apparently or di-s~bstituted29~ ineffective for all alkyl aldehydes. Despite the novelty of the reaction (Scheme l), its mechanism remains un- investigated, though it has long been recognized that the first step involves acid catalysed formation of a mixed anhydride of malonic and acetic An alternative-and unusual-source of Meldrum’s acid is from the action of carbon suboxide on acetone in the presence of oxalic acid.g The function of the latter seems to be to supply the elements of water needed for the stoicheiometry.1° 3 Physical and Chemical Properties By the standards of organic chemistry 1,3-dioxan-4,6-dione derivatives are indeed strong acids.Meldrum’s acid itself (pK 4.83)11 is comparable in strength to acetic acid (pK4.76) and is some ten pKunits more acidic than acyclic malonate esters. The explanation of this facile proton loss-as with dimedone (pK 5.2)2-lies in the stability of the resultant anion (5), in which the .rr-orbitals are rigidly held in the ideal configuration for overlap. The meneidicl2 properties of this profoundly stable structure rationalize much of Meldrum’s acid chemistry. Surprisingly, the tautomeric properties of dimedone and Meldrum’s acid are quite different. The former exists predominantly in the mono-enol form, whereas Meldrum’s acid is overwhelmingly ( > 99.5 %) the diketo tautomer.13 It seems that no special explanation may be required for this anomaly, since esters are generally enolized to a much smaller extent than ketones.l* The problem of the conformation of the 1,3-dioxan-4,6-dione system has been approached by n.m.r.15 and dipole-momentl6 methods.Although there is no E. Ott, Annalen, 1913, 401, 159. P. J. Scheuer and S. G. Cohen, J. Amer. Chem. SOC., 1958, 80, 4933. A. Michael and J. Ross, J. Amer. Chem. Soc., 1933, 55, 3684. 0. Diels, R. Beckrnann, and G. Tonnies, Annalen, 1924, 439, 76. lo T. Kappe and E. Ziegler, Angew. Chem., 1974, 86, 529; Angew Chem. Internat. Edn., 1974, 13, 491. l1 (a)K. Pihlaja and M. Seilo, Acfa. Chem. Scand., 1968,22, 3053; (b)K. Pihlaja and M. Seilo, Acta. Chem. Scand., 1969, 23, 3003. l2D. Lloyd and D. R.Marshall, Angew.Chem., 1972, 84, 447; Angew. Chem. Internat. Edn., 1972, 11, 404. l3 M. Eigen, G. Ilgenfritz and W. Kruse, Chem. Ber., 1965, 98, 1623. l4 A. Cero, J. Org. Chem., 1954, 19, 1960. l5 P. Ayrtiis and A. Partanen, Finn. Chem. Letters, 1976, 110. lo (a) D. Korberl and P. Schuster, Monatsh., 1972, 103, 1483; (6) D. Korberl and 0. E. Polansky, Monatsh., 1973, 104, 1421 ; (c) E. N. Klimovitskii, L. K. Yuldasheva, and B. A. Arbuzov, Izvesf. Akad. Nauk. S.S.S.R.,Ser. khim., 1973, 1527. McNab overall agreement as yet, the consensus of opinion favours a boat structure for most derivatives, with a chair structure for 2,2,5,5-tetrasubstitutedcom-pounds. This last conclusion is confirmed by the X-ray structure of an esoteric derivative.17 Other physical properties of the system which have been discussed include u.v.,~ i.r.,I* mass,l9 lH-,20 and 13C-n.m.r.21 spectra.The chemistry of Meldrum’s acid is dominated by its susceptibility to nucleo- philic attack at positions 4 and 6 and to electrophilic attack (via the anion) at position 5. Simple hydrolysis to the malonic acid is a common example of nucleo- philic attack, and may be accomplished under acidic or basic c0nditions.l The reaction mechanism has been studied.11 Use of alcoholic hydrogen chloride yields the malonate diester,3*4 while ‘solvolysis’ by phenols gives monoaryl esters which can be easily converted to diary1 esters.22 Ketones react with Meldrum’s acid by displacement of acetone to give 2,2-disubstituted-l,3- dioxan-4,6-diones.23 Nitrogen nucleophiles give a possible route to monoamides of malonic acid,24 although the cleavage of the ring may be followed by decarb- oxylation, as is the case with aniline,‘ and Hector’s base25 (Scheme 2).Frag-mentation of the ring may also be accomplished directly by pyrolysis. RNH, Scheme 2 In contrast to these examples, the reactions of Meldrum’s acid with electro- philes generally leave the ring intact. Scheme 3 gives the flavour of the chemistry involved, which is considered in detail in later sections. Reaction of electro- philes at the carbonyl oxygen does not give stable cyclic prod~cts.~~~~~~~ 4 5,5-Dialkyl Derivatives These derivatives may be made by standard condensation methods, but an important alternative route is by the reaction of Meldrum’s anion (5) with alky16 l7 P.G. Jones and 0. Kennard, Cryst. Struct. Comm., 1977, 6, 97. (a) R. A. Abramovitch, Canad. J. Chem., 1959, 37, 361; (6) E. E. Ernstbrunner, J. Mol. Structure, 1973, 16, 499. H. Egger, Monatsh., 1967, 98, 1245. eo I. Schuster and P. Schuster, Tetrahedron, 1969, 25, 199. 21 P. Ayras, Acta. Chem. Scand., 1976, B30, 957. 22 (a) H. Junek, E. Ziegler, U. Herzog, and H. Kroboth, Synthesis, 1976, 332; (b) G. Uray,H. Junek and E. Ziegler, Monatsh., 1977, 108, 423. *3 E. Ziegler, H. Junek, and H. Kroboth, Monatsh., 1976, 107, 317. 24 (a) C. D. Hurd and S. Hayao, J. Amer. Chem. SOC.,1954, 76, 5563; (6) P. Crooy, R. De, Neys, J. Eliaers, R. Liveyns, G. Simonet, and J. Vandevelde, Bull.SOC.chim. belges., 1977, 86, 991. 26 A. R. Butler, J. Chem. Res. (S), 1978, 50; (M),0855. S. I. Zav’yalov, Izvest. Akad. Nauk S.S.S.R., Otdel. khim. Nauk, 1961, 2042. G. A. Bihlmayer, P. Schuster, and 0. E. Polansky, Monatsh, 1966, 96, 145. 347 Meldrum’s Acid 0x0 0vo N* 0XOtT.:;,,NNHAr + SMe, Ac,O Scheme 3 (or activated heterocyclic28) halides [Scheme 3, equation (A)]. This almost in- variably29 gives the dialkyl derivative, and the procedure has been logically extended to the preparation of spiro compounds from dibrom~alkanes.~~ Since the formation of anions such as (5) cannot compete, hydrolysis of 5,5-disubstituted-l,3-dioxan-4,6-dionesbecomes almost instantaneo~s3~ under basic conditions. Malonate mono- and di-esters are also available, by standard methods.28 (a) F. X. Smith and G. G. Evans, Tetrahedron Letters, 1972, 1237; (6) F. X. Smith and G. G. Evans, J. Heterocyclic Chrm., 1976, 13, 1025; (c) F. X. Smith and A. Scoville, J. Heterocyclic Chem., 1977, 14, 1081. 29 R. B. Mane and G. S. K. Rao, Chem. and Ind., 1976, 786. 30 J. A. Hedge, C. W. Kruse, and H. R. Snyder, J. Org. Chem., 1961, 26, 992. 31 K. Pihlaja and J. Ketola, Finn. Chem. Letters, 1976, 123. McNab Of particular interest are the spiro cyclopropyl derivative~3~ e.g. (14) whose Michael-type reactions with nucleophiles are reminiscent of those of the 5-methylene compounds (Section 5). Intramolecular decomposition of the ring-opened product may lead to lac tone^^^.^^ (Scheme 4) or lac tarn^.^^ 5 5-Methylene derivative^^^ In some instances, reaction of a carbonyl compound (or its equivalent) with malonic acid proceeds directly by a double condensation to give 5-methylene derivatives of 1,3-dio~an-4,6-diones~~~~and a corresponding procedure using carbon suboxide is known.36 However, the preparative method of choice involves Knoevenagel condensation of Meldrum's acid with a carbonyl component [Scheme 3, equation (B)].This reaction proceeds easily for aromati~~~l~~ or hindered aliphatic aldehydeP or aliphatic ketone~,3~*~0 though aromatic ketones require activation either by the use of a catalyst,41 or by their prior conversion to a ketimine.42 Simple aliphatic aldehydes often give the 2:l condensation product, by Michael addition of Meldrum's acid to the intermediate 5-methylene compound3~3g-a process which is of paramount importance also in dimedone chemistry.In these examples, the 5-methylene compound is available by the cunning use of methoxide as a competitive nucleophile in the Michael addition. The adduct (6; Nu = OMe) so obtained may be decomposed with acid to the required product.43 The activation of a carbonyl compound by 0-methylation is particularly important when the resulting salt is resonance stabilized. Base catalysed con- densation of such cations with Meldrum's acid has afforded 5-methylene 32 (a)S. Danishevsky and R. K. Singh, J. Amer. Chem. SOC.,1975,97,3229; (b)S. Danishevsky and R. K. Singh. J. Org. Chem., 1975, 40, 3807; (c) R.K. Singh and S. Danishevsky, J. Org. Chem., 1976, 41, 1668. 33 F. P. Schmook and 0. E. Polansky, Monarsh., 1969, 100, 1631. 34 For a review, see F. J. Kunz, P. Mzrgaretha, and 0. E. Polansky, Chinria (Swirz.), 1970, 24, 165. 35 D. T. Mowry, J. Airfer. Chenr. SOC.,1947, 69, 2362. 38 H. Hopff and G. Hegar, Helv. Chim. Acta., 1961, 44, 2016. E. J. Corey, J. Amer. Chem. Soc., 1952, 14, 5897. 38 P. Schuster, 0. E. Polansky, and F. Wessely, Munatsh., 1964, 95, 53. 39 J. A. Hedge, C.W. Kruse, and H. R. Snyder, J. Org. Chem., 1961, 26, 3 166. ho G. Swoboda, J. Swoboda, and F. Wessely, Munatsh., 1964, 95, 1283. 41 G. J. Baxter and R. F. C. Brown, Austral. J. Chetn., 1975, 28, 1551. 42 G. A. Bihlrnayer, F. J. Kunz, and 0. E. Polansky, Munatsh., 1966, 97, 1293.43 P. Margaretha and 0. E. Polansky, Tetrahedron Letters, 1969, 4983. 349 Meldrum’s Acid pyryli~m,~Scompounds derived from cyclopr~penium,~~ and other hetero-cyclic46 salts. Eistert has made similar products from chloropyrylium salts,47 and even, in a peculiar redox reaction, from the unsubstituted pyrylium salt itself.48 0’XT,0XO 0x0 0x0 (9) The parent 5-methylene compound (7) is apparently highly reactive, and cannot be prepared by standard methods. The sequence which was ultimately successful (Scheme 5) employs a unique oxidative process in the key ~tep.~g 0XOoQ.o Me. SePh 8) Scheme 5 A number of functional derivatives of 5-methylene compounds are known. Thus (9; X = H, Y = OEt) is readily available from Meldrum’s acid and triethylorth~formate~~[Scheme 3, equation (C)], while addition of an amine to this reaction mixture gives the amino compounds (9; X = H, Y = NHR) in a one-pot synthesis of wide general it^.^^^^^ The diamino compound (9; X = Y = NHC6-H11), which is an extended urea, can be made from dicyclohexylcarbodi-imide,52 and the ‘amides’ (9; X = OH, Y = NHR) from is~cyanates.~~ A remarkable property of 5-methylene derivatives in general is that like the parent 1,3-dioxan-4,6-dione, they are unexpectedly strong a~ids.~~~~~ In this 44 T.Eicher, T. Pfister, and N. Kreuger, Org. Prep. Proc. Internat., 1974, 6, 63 (Chem. Abs.. 1974, 81, 3403). I6 (a)J. A. Van Allan and G. A. Reynolds, J. Hetrrucyclic Chem., 1971, 8, 803; (6) J.A. Van Allan and G. A. Reynolds, J. Heterocyclic Chem., 1972, 9, 669; (c) J. R. Wilt, G. A. Rey- nolds, and J. A. Van Allan, Tetrahedron, 1973, 29, 795. R. Neidlein and M. H. Salzl, Annalen, 1977, 1938. “B. Eistert and T. J. Arackal, Chem. Ber., 1975, 108, 2397. 4a B. Eistert, A. Schmitt. and T. J. Arackal, Chem. Ber.. 1976, 109, 1549. K.F. C. Brown, F. W. Eastwood, and G. L. McMullen, Austral. J. Chem., 1977, 30, 179. 50 G. A. Bihlmayer, G. Derfiinger, J. Derkosch, and 0.E. Polansky, Monatsh., 1967, 98, 564. 51 Sterling Drug Inc., Brit. 1,147,759 (Chem. Abs., 1969, 71, 70 125). 52 A. Stephen, Munatsh., 1966, 97, 695. j3 U. Herzog and H. Reinshagen, Eur. J. Mrrl. Cheni-Chim. Ther., 1975, 10, 323. .54 P. Schuster, A. Stephen, 0. E.Polansky, and F. Wessely, Monotsh., 1968, 99, 1246. 350 McNab case, it is a manifestation of Lewis acidity,55 due to the formation of pseudobase adducts or anbadons (6) for which the driving force is clearly the meneidic ten- dency of the stable anion. The whole range of hard and soft nucleophiles, from alkoxides55 to phosphines56 show this behaviour. The pseudobase may be quenched with iodomethane to give the alkylated products (10).40956 In some cases where the 5-methylene compound has a ,8-hydrogen available, proton loss (i.e. Bransted acid behaviour) can compete with pseudobase formation [e.g. (1111.57 Clearly the pseudobases (6; Nu = OH) are simply intermediates in the hydro- lysis of 5-methylene derivatives to Meldrum’s acid and the carbonyl component, and the kinetics and mechanism of this process have been ~tudied.~8 Under certain conditions, the heterocyclic ring may be preferentially hydrolysed, pro- viding a route to 2-methylenemalonate~.~~*~~Reduction of 5-methylene-l,3- dioxan-4,6-diones to the corresponding 5-alkyl compound is possible catalyti- cally,30 or using lithium aluminium h~dride.~~ Reaction with Fez(C0)g gives a n-cornplex760 while the labile formyl derivative (9; X = H, Y = OH), produced by hydrolysis of the ethoxy compound, gives a chelate with copper acetate.50 The 5-methylene derivatives of Meldrum’s acid are excellent dienophiles, and give a potential route to cyclohexenes with unusual substituents.49~61 This reaction has recently been exploited by two groups in natural product synthesis62963 and further developments in this area are awaited with interest. The 173-dipolar cycloaddition of dia~oalkanes~~has been studied in great detail.In all cases, cyclopropane derivatives are obtained, although the reaction may proceed twice65 (Scheme 6; R1 = P5, R2 = H) and traces of further alkylation can sometimes be detected.33 That the initial reaction is indeed a standard 1,3-dipolar cycloaddition, was proved by an elegant kinetic study,66 whose results were vindicated by the isolation of an intermediate pyra~oline~~ (Scheme 6; R1 = Pri, R = C02Et). 6 5-Halogeno Derivatives Although the 5-chloro compound has been prepared,G8 the known chemistry of 5-halogeno derivatives of Meldrum’s acid is almost wholly associated with brominated materials.Both the 5-bromo and the 5,Sdibromo compounds are formed by direct halogenation of Meldrum’s acid in the presence of one6 or 56 P. Schuster, 0. E. Polansky, and F. Wessely, Tetrahedron Suppl. 8 Part 2, 1966, 463. 58 P. Margaretha and 0. E. Polansky, Monatsh., 1969, 100, 576. 67 J. Leitich, P. Schuster, and A. Eitel, Tetrahedron, 1967, 23, 2221. 58 P. Margaretha, P. Schuster, and 0. E. Polansky, Tetrahedron, 1971, 27, 71. 68 A. Stephen and F. Wessely, Monatsh., 1967, 98, 184. Eo E. Korner von Gustorf, 0.Jaenicke, and 0. E. Polansky, Z. Natrirfursch. B., 1972, 27, 575. F. J. Kunz and 0. E. Polansky, Monatsh., 1969, 100, 920. 6z W. G. Dauben, A. P. Kozikowski, and W. T. Zimmermann, Tetrahedron Letters, 1975,515.83 G. A. Mock, A. B. Holmes, and R. A. Raphael, Tetrahedron Letters, 1977, 4539. O4 F. Wessely and A. Eitel, Monatsh., 1964, 95, 1577. 65 G. Swoboda, A. Eitel, J. Swoboda, and F. Wessely, Monatsh., 1964, 95, 1355. e6 F. Nierlich, P. Schuster, and 0. E. Polansky, Monatsh., 1971, 102, 438. 87 H. Kisch, F. Mark, and 0. E. Polansky, Monatsh., 1971, 102, 448. O8 0. Y. Neiland and B. Y. Karele, Zhur. org. Khim., 1971, 1674. Meldrum's Acid R' = Ph, R' = H: -70 C r 1 RTHN,0X O I 4 O 2R2 l RI 1 \ -J f RZ R' = Ph, R' = H:+ 20 C Scheme 6 moles of base respectively [Scheme 3, equation (D)]. A unique situation pertains to iodine-containing derivatives, since a phenyliodonium ylid is well characterized68 yet no classical iodo compound has ever been reported.Treatment of the 5-bromo-derivative (12; R = H) with nucleophiles can give rise to three possible modes of reaction (Scheme 7). The trivial case, in which UU + Br-Br R Br R Nu Scheme 7 the nucleophile acts as a base, is the dominant route even for such mild bases as aniline.6g The ubiquitous anion may also be formed by nucleophilic attack at the halogen atom (Scheme 7, route 2). This route is particularly important 69 H. R. Snyder and C. W. Kruse, J. Anirr. Chem. So<..,1958, 80, 1942. McNab for (12; R =Me), the use of which as a brominating agent for carbanions has been recommended.70 Trost’s process of ‘transfer alkylation’ is a subtle applica- tion of this principle, in which the anion leaving group re-attacks the brominated substrate at a remote site, to complete a chain extension sequence.71 Although there is no unambiguous example of a simple SNreaction from (12; R =H), this mechanism can apparently give rise to products with hard nucleophiles and (12; R =Me).72 The 5,5-dibromo compound (12; R =Br) is also a useful brominating agent,73 but little else is known of its chemistry save for two astonishing reactions.69 Basic hydrolysis gives carbon tetrabromide in a process which, for its total destruction of a stable system to C1 fragments is reminiscent of the bromopicrin reaction.74 Secondly, simple dissolution in dimethylformamide yields the coupled derivative (13) in 30%yield.The mechanisms of these reactions are overdue for investigation.x Ox0 oL)(Jo H NH2 (13) (14) 7 5-Nitrogen-containing Derivatives Although Davidson and Bernhard reported a colour reaction of Meldrum’s acid with sodium nitrite,2 it was not until 1961 that two groups independently isolated the oxime as an unstable yellow solid3926 [Scheme 3, equation (E)]. Its reduction over platinum dioxide provides the only known route to the 5-amino compound3 (,l4), but otherwise the properties of this compound are unknown. Similarly little studied are the hydrazones, prepared by coupling with appropriate diazo compounds75 or diazonium salts3 [Scheme 3, equation (F)]. The parent 4,5,6-trione of which these compounds are formal derivatives, remains un-prepared, despite a number of attempt~.~~*~~ The most important nitrogen-containing derivative of Meldrum’s acid is the diazo-compound (15), formed by diazotization of the amir~e,~ or, more con- veniently, by direct reaction with p-toluenesulphonyl azide75 [Scheme 3, equa-tion (G)].Photolysis or thermolysis of (1 5) generates ‘Meldrum’s ~arbene’~~ which io (a) J. P. Marino, J.C.S. Chem. Conzm., 1973, 861 ;(6) E. Vedejs, W. R. Wilber, and R. Twieg, J. Org. Chem., 1977, 42, 401. i1 B. M. Trost and L. S. Melvin jun., J. Amer. Chem. SOC.,1976, 98, 1204. i2 R. F. C. Brown, F. W. Eastwood, S. T. Lim, and G. L. McMullen, Austral. J. Chem., 1976, 29, 1705. 73 R. Bloch, Synthesis, 1978, 140. “A. R. Butler and H. F. Wallace, J. Chem. SOC.(B), 1970, 1758.75 (a) M. Regitz and D. Stadler, Annalen, 1965, 687, 214; (6) M. Regitz, A. Liedhegener, and D. Stadler, Annalen, 1968, 713, 101. 76 M. Regitz and H-G. Adolph, Annalen, 1969, 723, 47. 7i A. Hochrainer and F. Wessely, Monatsh., 1966, 97, 1. M. Jones Jr., W. Ando, M. E. Hendrick, A. Kulczycki jun., P. M. Howley, K. F. Hummel, and D. S. Malament, J. Amer. Chem. SOC.,1972,94,7469. Meldrum’s Acid may be trapped with soft nucleophiles to give ylide~,~~q~O or with olefins in the presence of triplet sensitizer to give the predicted trans cyclopropane derivatives.78 In the absence of trapping agent, the normal ketocarbene Wolff ring-contraction takes place (Scheme S), though under extreme conditions, the system may be totally destroyed, with acetone and carbon monoxide as dominant products.81 (15) Scheme 8 8 Ylides Meldrum’s acid ylides (1 6) of the sulphonium,77~79 selenonium,82 arsonium780 and iodonium68 series have been prepared.Their syntheses require either condensation of Meldrum’s acid itself with a highly oxidized hetero-component (possibly generated in sit^),^^ or the decomposition of the diazo-compound (1 5) in the presence of the hetero-component itself (Scheme 9). In addition, the 0x0 lor OX0 + R,,M0,+RnMX2 + 0 a0 -oXo 7oJ+o + MR, N* a; M = S, R = dialkyl, n = 2 (X = OAc) b; M = Se, R = Me, ti = 2, X = Br C; M = AS, R = Ph, n = 3 (X = OAC) d; M = 1, R = Ph,II = I, X = OAc Scheme 9 dimethylsulphonium ylid (16a) is available from the bromo compound77 (12; R = H) and from the iodonium ylid68 (16d).The most noteworthy property of these ylides is their remarkable stability, inevitably associated with the dipolar canonical form (16). All are crystalline solids which may be handled without special precautions. The dimethylsulphonium and phenyliodonium ylides (16a) and (16d) show 78 W. Illger, A. Liedhegener, and M. Regitz, Annalen., 1972, 760, 1. I. Gosney and D. Lloyd, Tetrahedron, 1973, 29, 1697. slS. L. Kammula, H. L. Tracer, P. B. She\lin, and M. Jones jun., J. Org. Chem.,1977, 42, 293 1. 8a E. Ernstbrunner and D. Lloyd, Annalen, 1971, 753, 196. 83 J. I. G. Cadogan and 1. Gosney, JCS Perkin I, 1974, 466. McNab pronounced thermal stability by comparison with related systems, while the stability of the selenonium and arsonium ylides (16b) and (16c) is reflected in their lack of reactivity with aldehydes in attempted Wittig reactions.The iodonium ylid (1 6d) gives 5-chloro-l,3-dioxan-4,6-dioneon treatment with hydrogen chloride.68 9 Pyrolysis The pyrolysis of Meldrum's acid derivatives has long been known to give ketenes.6 The mechanism presumably involves homolysis of a C-0 bond, followed by cleavage of acetone and carbon dioxide, both of which are found in the pyrolysate along with the ketene.84 (Scheme lo). Using flash vacuum L, y +co,+ RyR0 IIoJ$o 0 RR Scheme 10 pyrolysis, this route may be used to prepare ketenes which are difficult to obtain by conventional met hods-no tab1 y cyclopropyl ke tenes5-bu t the technique has been applied most particularly to the generation of methyleneketenes from 5-methylene derivatives.Thus phenylmethyleneketene (1 7) has been identified by its i.r. spectrum at -196"C,by its dimerization to the cyclobutandione deriva- tive (18), and by various trapping experimentss4 (Scheme 11). Methyleneketenes 0 Scheme 11 are available thus, in two steps, from aryl aldehydesS4 and ketoness6, or alkyl ketones9 those derived from aliphatic aldehydes tautomerize if possible to vinyl ketenes.84 Other more interesting intramolecular reactions which may take place in the presence of a suitable trapping group, include an elegant phenol 84 R. F. C. Brown, F. W. Eastwood, and K. J. Harrington, Austral. J. Chem., 1974, 27,2373.G.J. Baxter, R. F. C. Brown, F. W. Eastwood, and K. J. Harrington, Tetrahedron Letters, 1975, 4283. R. F. C. Brown, F. W. Eastwood, K. J. Harrington, and G. L. McMullen,Austra/.J. Chem., 1974, 27,2393. Meldrum’s Acid synthesiss7 (Scheme 12). Recently, the parent methyleneketene has been identified from the pyrolysis of the very reactive compound (7), itself generated in sit[/ by thermal retrocyclization from the adduct (S)49 (Scheme 5). 0 OHA6 q)E!!5+Q)A II 4 0% Scheme 12 Pyrolysis of Meldrum’s acid derivatives at high temperatures causes decar- bonylation of the intermediate ketene to give a carbene, which then rearranges to valence-satisfied products. Reactions of this type include the generation of acyloxycarbenes and their rearrangement to a-diketones in excellent yield (Scheme 13).72 Similarly, methylenecarbenes generated from methylneeketenes I co R’ Scheme 13 show hydrogen migration to give acetylenes, although intramolecular cycliza- tion products may also be detected in favourable instances.B6 10 Some Synthetic Uses As with simple malonic ester syntheses, the presence in Meldrum’s acid of a highly active methylene group adjacent to an ester function, allows the molecule to be manipulated to give a wide range of possible products.At the simplest level, Meldrum’s acid can act as a methylene synthon by hydrolysis and exhaustive decarboxylation of its derivatives. For certain heterocyclic systems, the conditions needed for this sequence are surprisingly mild28*45 (Scheme 14).Intermolecular cleavage by other nucleophiles can also give useful products; a particularly beautiful example is Dauben’s two step synthesis of 6-damascone62(19) Scheme 15). This is the only known example of a potentially general ketone synthesis from Meldrum’s acid. We have already met examples of intramolecular nucleophilic attack (Scheme 4). IJnder extreme conditions, cleavage of acetone may be followed by decarboxylation to give lactamss8 (Scheme 16); in these cases, Meldrum’s acid acts as the synthetic equivalent of ketene. 87 (a)R. F. C. Brown and G. L. McMullen, Austral. J. Chem., 1974, 27, 2385; (6) G. J. Baxter, R. F. C. Brown, and G. L. McMullen, Airstral. J. Chetn.. 1974, 27, 2605. 88 G.Y. Lesher, U.S. 3,907,798 (Chrm.Ah., 1976, 84, 44130). 356 McNab AcOHIHCIO, 100,lo min Ar Ar02:r Scheme 14 Scheme 15 0x0 H Scheme 16 0 0 OH XO 0Uo+ mo Scheme 17 357 Meldrum’s Acid Finally, a widespread use of Meldrum’s acid in heterocyclic chemistry is in the preparation of a-pyrones and 2-pyridones. A variety of such reactions is known, and some are of formidable complexity, but a common feature is that the Meldrum’s acid behaves as a C302 synthon45~89~90(Scheme 17). I am grateful to Professor J. I. G. Cadogan for his encouragement, and to Dr. D. Lloyd for his invaluable comments on the manuscript. 89 (a) E. Ziegler, H. Junek, and U. Herzog, Monatsh, 1971, 102, 1096; (6) E. Ziegler, H. Junek and U. Herzog, Monatsh, 1971, 102, 1626. Bo 0. S. Wolfbeis, Monarsh, 1977, 108, 499.