The chemistry of thujone. IX. Thujone as a chiral synthon for the synthesis of optically active steroid analogues. The vinylpicoline route

The thujone-derived enone 1 , upon base-catalyzed reaction with 2-methyl-6-vinylpyridine is converted to the pyridine analogue 5 (Scheme 1). Catalytic reduction of the latter to 6 generates two new centers of chirality which eventually become C(8) and C(14) in the ultimate synthetic steroid analogue 12 . An X-ray analysis of 6 establishes the structure and absolute configuration so as to determine its suitability in subsequent synthetic studies. The acetal derivative 7 , via Birch reduction, hydrolysis, and internal aldol cyclization, is converted into the cyclohexenone analogue 10 (Scheme 2). This ‘one-pot’ process affords an efficient conversion of the pyridine ring into a cyclohexenone system required for A-ring construction of the steroid skeleton. Finally, conversion of 10 , via the unsaturated diketone 11 , provides the chiral steroid analogue 12 .     

Model for rebound reactions: M+RI

A simple model, consisting of ‘two-body’ reactant and product trajectories joined by an instantaneous switch, is proposed for the alkali atom-alkyl iodide rebound reactions. The electronic potential surfaces governing these trajectories are justified in terms of the electron jump model. Encouraging overall agreement is found with extensive molecular beam data for the K, Cs + CH₃I reactions.     

Synthesis of C-9 oxidised N-acetylneuraminic acid derivatives as biological probes

Sialic acids are 9-carbon acidic sugars involved in a number of important biological processes and human diseases. As part of our ongoing interest in the development of novel sialic acids as biological probes, we have developed an efficient and simple synthesis of C-9 oxidised sialic acid derivatives. The key oxidative step involves the use of TEMPO under carefully controlled aqueous pH conditions.     

Elastic scattering of alkali dimers : rainbow structure for K2+CCl4, C6H12

Ab initio calculations for the t-butyl radical, (CH₃)₃C· suggest that it is planar, and that it has a greater resistance to out-of-plane bending than has the methyl radical. The calculated isotropic hyperfine coupling to ¹³C is +36·8 G for the rigid molecule, together with +32 G for the zero-point vibratory contribution, giving a predicted total of +68·8 G. The experimental isotropic coupling of 46–51 G cannot, therefore, be taken as providing evidence for a pyramidal ground state for this radical.     

Studies in the synthesis of 7,8-benzo[5]metacyclophane

Compound 5 reacted with silver perchlorate in anhydrous benzene to give 3a and in wet acetone to give a mixture of 9 and 10; treatment of 9 with potassium t-butoxide in dimethyl sulphoxide gave 11.     

Dispiroketals in synthesis (Part 5): A new opportunity for oligosaccharide synthesis using differentially activated glycosyl donors and acceptors

The reactivity of dispiroketal protected thioglycosides makes them useful new precursors for oligossaccharide synthesis as is illustrated by the preparation of a protected pentasaccharide unit common to the variant surface glycoprotein of Trypanosoma brucei.     

Reactive scattering of a neon seeded oxygen atom beam

A high pressure microwave discharge source operating with a dilute mixture of O₂ in Ne has been used to produce a supersonic nozzle beam of O atoms seeded in Ne. This low energy supersonic O atom beam has been used to study the reactive scattering of O atoms with Cl₂ and CS₂ molecules at an initial translational energy E = 13 kJ mol^?1. The results are compared with rective scattering from the same reactions using a high energy O atom beam formed by seeding O atoms in He. The O + Cl₂ reaction proceeds via a short-lived collision complex where the lifetime of the collision complex depends upon the initial translational energy. However the O + CS₂ reaction follows a stripping mechanism which is unaffected by initial translational energy.     

Reactive scattering of a helium seeded oxygen atom beam

A high pressure microwave discharge source operating with a dilute mixture of O₂ in He has been used to produce a supersonic nozzle beam of O atoms seeded in He. This source has been used to study the reactive scattering of O atoms with Cl₂ and CS₂ molecules at an initial translational energy E = 38 kJ mol^?1. Velocity distribution of reactive scattering were measured over a wide angular range by cross-correlation time-of-flight analysis. The O + Cl₂ reaction proceeds via a short-lived collision complex while the O + CS₂ reaction follows a stripping mechanism.