摘要:

On Dien‐Ketenes fromo‐Quinol‐AcetatesA detailed picture of the photochemistry ofo‐quinol‐acetates is presented. (RS)‐6‐Acetoxy‐6‐methyl‐, (RS)‐6‐acetoxy‐2,6‐dimethyl‐, (RS)‐6‐acetoxy‐5,6‐dimethyl‐, (RS)‐6‐acetoxy‐2,4,6‐trimethyl‐, (RS)‐6‐acetoxy‐2,3,4,6‐tetramethyl‐, and (RS)‐6‐acetoxy‐2,3,4,5,6‐pentamethyl‐2,4‐cyclohexadien‐1‐onesserve as representative educts. There are two separate main photochemical routes conveniently designated as1(π*, n) or3(π*, π) tracks. The latter may also be attained by sensitization and leads to phenols. The former, by α‐cleavage furnishes dienketens as indispensable phototransients. Photolysis of dien‐ketens follows one or more of three reaction channels, each of which yields a particular type of photoproduct: heat‐induced monocyclization affords 2,4‐cyclohexadien‐1‐ones, heat‐induced bicyclization stereoselectively furnishes bicyclo[3.1.0]hex‐3‐en‐2‐ones, and multi‐step addition of protic nucleophiles stereoselectively gives 1,4‐, 1,6‐ and/or 1,2‐adducts. By X‐ray analysis or NOE studies, the structure of isolated photoproducts is established. Conventional spectroscopy at low or flash spectroscopy at normal temperature yield information on the formation and decay of kinetically unstable intermediates. Photoproduct composition depends on the pattern of substitution of the educts, on the solvents, and on the nucleophiles that might be present. Substituents primarily exert an influence upon the population of the various conformers of the dien‐keten. Solvents affect the rate of the divers reaction paths competing for the phototransient. Nucleophiles play more than a trivial role when adducts are formed. With the detailed view of a dien‐keten's role on hand, the pho

ISSN:0018-019X    

DOI:10.1002/hlca.19860690302

出版商:WILEY‐VCH Verlag GmbH    

年代:1986

数据来源: WILEY

摘要:

AbstractA novel highly substrate‐specificRauwolfiaenzyme, raucaffricine β‐D‐glucosidase, was isolated from cell‐suspension cultures ofR. serpentina. The enzyme has been purifiedca.1350‐fold, its major characteristics such asMr= 66600 ± 5%, pH optimum 5.1, temperature optimum 38°, and inhibition of its activity by glucose and fructose were investigated. Its limited distribution in different cell cultures and differentiated plants indicates that the enzyme is present in significant amounts exclusively in culturedRa

ISSN:0018-019X    

DOI:10.1002/hlca.19860690303

出版商:WILEY‐VCH Verlag GmbH    

年代:1986

数据来源: WILEY

摘要:

AbstractThe first syntheses of 9‐bromo‐ and 9‐chlorobicyclo[4.2.1]nona‐2,4,6‐trienes were each achieved in three stereoselective steps from the 9,9‐bis(selenophenyl) derivative9in 79% overall yield for the bromide and 64% for the chloride. A deuterium‐labeling experiment reveals the first rearrangement of a bicyclo[4.2.1]nonatriene which leaves the ring

ISSN:0018-019X    

DOI:10.1002/hlca.19860690304

出版商:WILEY‐VCH Verlag GmbH    

年代:1986

数据来源: WILEY

摘要:

AbstractOn π,π*‐excitation of the epoxyenone (E)‐1(λ = 254 nm, MeCN), in addition to the previously isolated compounds2–9, the new products10–12, derived from the ylide intermediatecwere isolated. Further evidence for the ylidecwas obtained by the rapid racemization of the optically active epoxyenone

ISSN:0018-019X    

DOI:10.1002/hlca.19860690305

出版商:WILEY‐VCH Verlag GmbH    

年代:1986

数据来源: WILEY

摘要:

The Gas‐Flow Thermolysis of 1‐Isobutenyl Alkynyl and 2‐Methylphenyl Alkynyl Ketones. Synthesis of Methylenomycin BThe gas‐flow thermolysis of 1‐isobutenyl alkynyl or 2‐methylphenyl alkynyl ketones were found to lead to phenols and cyclopentenones or to naphthols and indanones, respectively. These conversions involve two cyclization processes so far unknown with α‐alkynones; they are interpreted as intramolecular additions of an allylic or a benzylic C,H bond to a triple bond which may occur in two directions. In addition, the cyclopentenones formed by the α‐alkynone cyclization, a known carbene process yielding 5‐rings, were also found. The available evidence ruled out a carbene process yielding 6‐rings. The addition process yielding 5‐rings was applied to a short (but low yield) synth

ISSN:0018-019X    

DOI:10.1002/hlca.19860690306

出版商:WILEY‐VCH Verlag GmbH    

年代:1986

数据来源: WILEY

摘要:

AbstractThe synthesis of a series of phosphine‐functionalised macrocycles,1–6, is described. The combination of N‐and O‐sites with P‐ and S‐sites provides ligands which may bind transition or non‐transition metal ions; as a consequence, they give access to dinuclear complexes containing both aLewisacid and a redox metallic site. Compounds1,2and6are heterodinucleating ligands capable of binding two dissimilar metals in proximity. Macrocycles3–5are homotopic ligands which may form homodinuclear complexes of tr

ISSN:0018-019X    

DOI:10.1002/hlca.19860690307

出版商:WILEY‐VCH Verlag GmbH    

年代:1986

数据来源: WILEY

摘要:

AbstractThree macrocyclic hexaamines1,2, and4, and the acyclic tetraamine5and hexaamine6have been synthesized. The hexaamines1,2, and4are ditopic coreceptor molecules containing two triamine subunits which may bind anionic substrates when protonated. The stability constants of the complexes between the protonated forms of the macrocyclic polyamines and terminal dicarboxylates−O2C−(CH2)m‐ CO2−as well as amino‐acid and dipeptide dicarboxylates have been determined by pH‐metric measurements. Around neutral pH,1and2give mainly complexes of the fully protonated species1·6H+and2·6H+, whereas4yields predominantly complexes of4·5H+and4·4H+. The stability sequences of the complexes formed indicate preferential binding of the dianionic substrates whose length is compatible with the separation of the triammonium binding subunits in the protonated receptor molecules1,2, and4. This selectivity pattern corresponds to a process oflinear molecular recognitionbased on ditopic binding between the two ammonium subunits of the coreceptor and the terminal carboxylates of the substrate of complementary length. The complexes of the acyclic ligands5and6are much weaker and much less selective, indicating a marked macrocyclic effect on both stability and selectivity of binding,i.e

ISSN:0018-019X    

DOI:10.1002/hlca.19860690308

出版商:WILEY‐VCH Verlag GmbH    

年代:1986

数据来源: WILEY

摘要:

AbstractUnder carefully controlled conditions, boron trichloride or alkoxydichloroborane/ethyldiisopropylamine in CH2Cl2can be used to effect diastereoselective aldol additions of ethyl ketones to saturated, α, β‐unsaturated, or aromatic aldehydes. The CC bond formation takes place with relative topicityul(‘syn,’ configuration of the aldols), in selectivities ranging from 90 to 99% ds (Tables 1–3). Mechanistic aspects of the reaction ar

ISSN:0018-019X    

DOI:10.1002/hlca.19860690309

出版商:WILEY‐VCH Verlag GmbH    

年代:1986

数据来源: WILEY

摘要:

AbstractThe α‐hydroxylation of chiral esters of 3‐phenylpropionic acid by MoO5·Py·HMPT was optimized to 98% de and 73% yield by systematic variation of the reaction conditions. The addition of at least 3 equiv. of K(sec‐BuO) proved to be e

ISSN:0018-019X    

DOI:10.1002/hlca.19860690310

出版商:WILEY‐VCH Verlag GmbH    

年代:1986

数据来源: WILEY

摘要:

AbstractThe synthesis of the ansa chain of rifamycin S (1), corresponding to the C(19) to C(27) moiety, epimeric at C(23), is described starting from dimethylxylo‐3‐hydroxy‐2,4‐dimethylglutarate (4). The diester4was transformed to the C10‐diester16by twothreo‐aldol condensations with 2,6‐dimethylphenyl p

ISSN:0018-019X    

DOI:10.1002/hlca.19860690311

出版商:WILEY‐VCH Verlag GmbH    

年代:1986

数据来源: WILEY