摘要:

AbstractUltrasound can be used in chemistry to increase both reaction rates and yields of products. Most effects of ultrasound on chemical reactions are due to cavitation: the formation and collapse of small bubbles in the solvent. In this review, we first outline the physical background of cavitation, and discuss its dependence on factors such as sound intensity and frequency, solvent and temperature. The impact of ultrasound on chemical reactions is considered for homogeneous reactions and for heterogeneous liquid‐solid systems. The first area is mainly illustrated by a discussion of the effect of ultrasound on polymerization and depolymerization reactions, the second by selected examples in organic synthesis. The tendency of ultrasound to change reaction mechanisms in favour of homolytic (instead of heterolytic) pathways, is also briefly discussed. The specific preference for a particular pathway under sonochemical conditions, different from that under mechanical stirring has been termed “sonochemical switching”. Ultrasonic equipment for lab‐scale experiments are compared, and some practical “tricks and traps

ISSN:0165-0513    

DOI:10.1002/recl.19941131202

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractResearch on endohedral fullerenes has yielded unambiguous evidence for the existence of selected inside‐the‐cage complexes. Milligram quantities of purified samples of metallofullerenes have been produced, and characterized via techniques that selectively probe the endohedral guest. Experiments aimed at a detailed understanding of the molecular properties of the endohedral complexes as well as of the material properties of compounds containing endohedral fullerenes have just star

ISSN:0165-0513    

DOI:10.1002/recl.19941131203

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractHeteroconjugate addition of various alkyllithium reagents to α‐silylallenephosphonates3b,chas been accomplished in high yields. The formation of thiophene‐2‐phosphonates 5, by heteroconjugate addition of organolithium to3bc, followed by treatment with sulfur dioxide, proceeds in reasonable yields via the intermediate formation of α,β‐unsaturated sulfines4. An alternative synthesis of the heteroconjugate addition product diethyl 1‐(trimethylsily)‐2,3‐dimethylbut‐2‐ene‐1‐phosphonate6afrom 2,3‐dimethylbut‐2‐enyl b

ISSN:0165-0513    

DOI:10.1002/recl.19941131204

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe all‐Eisomers of (12,12′‐13C2)‐, (13,13′‐13C2)‐, (14,14′‐13C2)‐, (15,15′‐13C2)‐ and (20,20′‐13C2)astaxanthin were prepared by total synthesis starting from commercially available 99%13C‐enriched acetonitrile, acetic acid and methyl iodide. The astaxanthins were obtained in high purity and with high isotope incorporation (>99% for every position). For this synthesis, the C15+ C10+ C15strategy was used. The central C10‐synthon, 2,7‐dimethylocta‐2,4,6‐triene‐1,8‐dial, was symmetrically dilabelled at any position using two new synthetic schemes. The13C2‐enriched C10‐dialdehydes were then convert

ISSN:0165-0513    

DOI:10.1002/recl.19941131205

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe synthesis of two new analogues (i.e.3and4) of farnesyl pyrophosphate (FPP) containing a phosphonophosphate and a phosphonophosphonate moiety was accomplished via the phosphonomorpholidate. The latter was easily obtained by treatment of a phosphonic chloride with morpholine.

ISSN:0165-0513    

DOI:10.1002/recl.19941131206

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractChlorozinc enolates of (hetero)aryl‐, alkyl‐ and silyl‐substituted acetates were reacted with imines and α‐diimines. The zinc‐mediated reaction of (hetero)aryl‐substituted acetates with imines affordedtrans‐β‐lactams in good yields and with excellent selectivity. Zinc enolates of non‐functionalized alkanoates yielded the corresponding 3‐alkyl‐β‐lactams (predominantlytrans) in moderate yields. Bulky substituents increase the rate and enhance thecis‐transselectivity of the reaction. Methyl 2‐(trimethylsilyl)acetate does not afford 3‐(trimethylsilyl)‐β‐lactams via the zinc‐mediated reaction with imines, butN‐(trimethylsilyl)‐β‐amino esters, which are formed via an intramolecular silyl shift. The desilylated β‐amino esters were obtained from the zinc enolate of methyl acetate, albeit in moderate conversion, and careful control of the reaction conditions was necessary to avoid side‐reactions. The substituent effects are discussed in terms of pre‐equilibria involving

ISSN:0165-0513    

DOI:10.1002/recl.19941131207

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractNatural abundance15N‐NMR spectra of poly(propyleneimine) dendrimers are presented to show the usefulness of the technique in characterizing these highly branched and well‐defined macromolecules. Inverse‐gated decoupled measurements show the number and intensities of signals that reflect the generations in the dendr

ISSN:0165-0513    

DOI:10.1002/recl.19941131208

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractEssentially complete fission of the P‐P bonds in red phosphorus can be effected by adding one equivalent oft‐butylalcohol to a mixture of red P and lithium in liquid ammonia. The reaction has been applied to synthesize some dialkylphospha

ISSN:0165-0513    

DOI:10.1002/recl.19941131209

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

ISSN:0165-0513    

DOI:10.1002/recl.19941131210

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

ISSN:0165-0513    

DOI:10.1002/recl.19941131201

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY