Dhanonjoy Nasipuri

Cyclisation reactions. Part 5. Synthesis of pallescensin A, a furanoid sesquiterpene by cationic cyclisation of an ω-furyldiolefin and of the corresponding epoxy-olefin

A biomimetic synthesis of pallescensin A (1), a furanoid sesquiterpene of natural origin is effected by concerted cyclisation of (6E)-9-(3-furyl)-2,6-dimethylnona-2,6-diene (2). The diene (2) has also been converted into a mono-epoxide (3) and the latter cyclised stereoselectively into 3-β-hydroxypallescensin A(4) which is subsequently deoxygenated to (±)-pallescensin A.

Asymmetric synthesis. Part 6. Asymmetric reduction of aminoketones with (–)-bornan-2-exo-yloxyaluminium dichloride

α-Dialkylaminoacetophenones and β-dialkylaminopropiophenones have been reduced asymmetrically with (–)-bornan-2-exo-yloxyaluminium dichloride to the corresponding aminoalcohols in 58–92% enantiomeric excess. The absolute configurations of the predominant enantiomers, (S) and (R) for α- and β-series respectively, follow from six-membered cyclic transition states. Three acetylpyridines have been similarly reduced, but with much less enantioselectivity (12–37%).

Stereoselective synthesis of (±)-veadeiroic acid and (±)-veadeirol by cyclisation of a 2-(2-arylethyl)-1,3,3-trimethylcyclohexyl cation: mechanism and stereochemistry of related cycloalkylation reactions

(±)-Veadeiroic acid 1 and (±)-veadeirol 2, two new cleistanthoid diterpenes, have been synthesized by stereoselective cyclisation of the carbocation 5 generated from the cyclohexanol 26. The latter is prepared from 2-ethyl-3-formylbenzoic acid (with CO2H masked as an oxazoline ring) by successive reaction with 3-methylbutan-2-one, Robinson annutation, and reduction of the styrylcyclohexenone 24. The stereochemistry of cycloalkylation of the carbocations A, which ranges from 100%trans to 50 : 50 cis-trans ring-fusion as a non-activated aryl ring becomes activated by substitution, has been rationalised on the basis of the shift of the rate-determining step from the formation of σ-complexes to that of π-complexes and the steric interactions therein.

Polycyclic systems. Part 19. Synthesis of 8-isobutyl-10-methyl-11H-indeno[2,1-a]phenanthrene (second diels hydrocarbon), a minor dehydrogenation product of cholesterol

The structure of the second Diels hydrocarbon, a dehydrogenation product of cholesterol, has been confirmed as 8-isobutyl-10-methyl-11H-indeno[2,1-a]phenanthren(1) by an unambiguous synthesis. 4-Isobutyl-2-methyl-1-bromobenzene (5) prepared from m-acetotoluidide was converted by a series of standard reactions into 2,4-dimethyl-6-isobutylindan-1-one (10) which on condensation with 2-(1-naphthyl)ethylmagnesium bromide followed by cyclisation and dehydrogenation afforded the desired hydrocarbon (1), identical with the second Diels hydrocarbon in all respects.

Asymmetric reduction of carbonyl compounds with chiral alkoxyaluminium and alkoxymagnesium halides: an overview

A summary of the work on asymmetric reduction of carbonyl compounds with chiral alkoxyaluminium and alkoxymagnesium halides derived mainly fromexo- andendo-bornan-2-ols done in the present laboratory has been given. The highlights of the contributions are as follows: (i) the development of a new class of extremely enantioselective and diastereoselective reducing reagents which are easily accessible, (ii) practical synthesis ofα-deuterated benzyl alcohol, 2,2,2-trifluoro-1-phenylcthanol (a chiralnmr solvent) and some aminoalcohols of physiological importance in high chemical and optical yields, (iii) a new chemical method to determine the configuration of chiral ketones and (iv) a highly satisfactory transition state model for these reactions which explains all the literature data so far.

Restricted rotation. Part 3. Barrier to rotation in methyl 2-(8-quinolyl)-6-oxocyclohex-1-enylacetate: comparative study of the effective bulk of a nitrogen lone pair in quinoline and of a naphthalene hydrogen

The rotational free energy barrier of methyl 2-(8-quinolyl)-6-oxocyclohex-1-enylacetate (2) about the aryl–cyclohexenone bond is found to be 75.7 kJ mol–1 by dynamic n.m.r. The value is lower than that (95.4 kJ mol–1) for the corresponding 1-naphthyl derivative (1) indicating that the effective bulk of the nitrogen lone pair in quinoline is less than that of a naphthalene hydrogen. A similar study with methyl 2-(1-naphthyl)-5-oxocyclopent-1-enylacetate (4) shows that the energy barrier in this case is lower than 37.0 kJ mol–1.