Yoram Houminer

Efficient preparation of enol carbonates by selective O-acylation of ketone sodium enolates generated in the presence of TMEDA

Abstract Generating sodium enolates in the presence of TMEDA followed by inverse addition to chloroformate esters leads cleanly to the O -acylated materials and provides a practical means of preparing such derivatives. In the absence of TMEDA mixtures of O - and C -acylated materials are obtained.

Preparation and thermolysis of cis- and trans-1-hydroxy-2-(2-pyridyl)cyclopentanes and cis- and trans-1-hydroxy-2-(2-pyrazyl)cyclopentanes

Syntheses respectivement a partir des oxyde-1 de pyridine et morpholino-1 cyclopentene ou chloro-2 pyrazine et oxo-2 cyclopentylure; pyrolyses dans le diglyme

Vinylic cations from solvolysis. Part XV. β-Phenyl and β-p-methoxyphenyl rearrangement during the solvolysis of 2,2-diaryl-1-phenylvinyl bromides

Triphenylvinyl bromide (1), trans- and cis-2-p-methoxyphenyl-1,2-diphenylvinyl bromides (2) and (3), and 2,2-bis-p-methoxyphenyl-1-phenylvinyl bromide (4) were solvolysed in 2,2,2-trifluoroethanol (TFE) and in 60% EtOH. In TFE, the products were the trifluoroethyl ether from (1), 15% of a 1 : 1 cis–trans mixture of the unrearranged and 85% of the β-phenyl rearranged trifluoroethyl ethers from (2) and (3), and a 1 : 1 cis–trans mixture of the β-p-methoxyphenyl rearranged trifluoroethyl ethers from (4). With toluene-p-thiolate ion, (4) gave only a rearranged sulphide, whilst the sulphides from (2) and (3) are 70% rearranged. The trifluoroethanolysis of (1)–(3) was accompanied by common ion rate depression. The relative initial k1(120°) values were 1(1) : 3·4 (2) : 3·7 (3) : 9·5 (4). In 60% EtOH the products were mostly the unrearranged acetophenone derivative from (2) and (3) and only the rearranged ketone from (4), and the relative k1(140°) values were 1 (1) : 2·14 (2) : 2·44 (3) : 5·45 (4). It is concluded that (a) the initial ionisation does not involve β-aryl participation, (b) the products are derived from free, open, linear vinyl cations, which return to covalent bromide (kBr), rearrange (kr), and react with the solvent (kROH) or toluene-p-thiolate ion (kSR′), (c)β-p-methoxyphenyl (An) rearrangement is the dominant process for the ion An2CCPh in the two solvents, (d)β-phenyl rearrangement is dominant for the ion AnC(Ph)CPh (19) in TFE (kBr : kr(Ph) : kROH= 214 : 5·7 : 1 at 120°) while in 60% EtOH kROH:kr(Ph)= 19 : 1; the apparent ‘abnormal’ order of migratory aptitudes Ph > An is either due to a hidden β-p-methoxyphenyl rearrangement or to a higher energy loss in the deconjugation of the migrating aryl group with the double bond than the energy gain in the briding by the migrating aryl, and (e) the effects of β-aryl substituents on the solvolysis rate are nearly additive.

Intramolecular catalysis. Part II. Electrophilic anchimeric assistance by a hydroxy-group in the opening of steroidal epoxides by azide anions

5α,6α-Epoxycholestane and its 3-substituted derivatives and 4α,5α-epoxycholestane and its 7-substituted derivatives have been prepared and their structures established. The stereochemistry of epoxidation of 7-substituted cholest-4-enes and 3-substituted cholest-5-enes with m-chloroperbenzoic acid is discussed in detail. The reactions of the 4α,5α- and 5α, 6α-epoxides with sodium azide in refluxing acetone–water (2 : 1) gave the corresponding trans-diaxial hydroxy-azides. The presence of a 7α-hydroxy-group in a 4α,5α-epoxycholestane, and of a 3α-hydroxy-group in a 5α,6α-epoxycholestane, strongly accelerated the opening of the epoxide ring by the nucleophile. Evidence is given for an intramolecular electrophilically assisted reaction, and various factors which may affect the mechanisms of these reactions are discussed.

Intramolecular catalysis. Part III. Effect of a neighbouring hydroxy-group on the opening of steroidal aziridines with azide anions.

5α,6α-Iminocholestan-3β-ol (IV) and its 3α-hydroxy-isomer (V) have been prepared and their structures established. Their reactions with sodium azide in acetone–water (2 : 1) produce the corresponding trans-diaxial amino-azides. The ratio of the reaction rates is ca. 1 : 2, respectively. The mechanisms of the reactions of the steroidal aziridines are discussed and comparison is made between these compounds and the related epoxides.

Neighbouring hydroxy-group participation in the reductive elimination of chlorine from 5α,6β-dichlorocholestanes with sodium borohydride

Sodium borohydride-promoted, reductive elimination of chlorine from 5α,6β-dichlorocholestane and some of its derivatives has been studied in propan-2-ol. The presence of a 3α-hydroxy-group in the steroidal skeleton strongly accelerates the process, and evidence is given for an intramolecular, electrophilic, assisted reaction. Various mechanisms for the reductive elimination are discussed and an E2 mechanism is proposed for the reaction of the 1,2-dichlorides.