Revuru Venkateswarlu

New lignans from the heartwood of cleistanthus collinus

Abstract The isolation and characterisation of five new compounds and nine known compounds from the heartwood of Cleistanthus collinus are reported. The new compounds are wodeshiol 16 ,3,4-dihydrotaiwanin C 20 , and three new glycosides 24 , 25 and 26 of diphyllin and taiwanin E The 1 H and 13 C NMR spectra of these compounds are also reported.

Oxidative transformations of lignans - reactions of dihydrocubebin and a derivative with DDQ

Abstract Treatment of dihydrocubebin (1) with DDQ in acetic acid gives the 2-aryltetrahydrofuran (3) while with DDQ in trifluoroacetic acid it affords the isomeric dibenzocyclooctadiene (5). Treatment of the 3,4-dibenzyltetrahydrofuran (2), obtained by cyclisation of (1), with DDQ in acetic acid gives a mixture of the acetoxy compound (8) and the aryl tetralin (9), while with DDQ in trifluoroacetic acid it gives the dibenzocyclooctadiene (10). The structural elucidation of these products is described and mechanisms for their formation are presented.

Rearrangements of 2,6-diaryl-3,7-dioxabicyclo[3.3.0]octane lignans reactions of Paulownin and Wodeshiol with triethylsilane and BF3-etherate

Abstract Paulownin on treatment with triethylsilane and BF 3 -etherate reacts in a similar manner to gmelinol, giving an aryltetralin as the product. In contrast, wodeshiol under the same conditions rearranges to two isomeric tetrahydropyran derivatives, the structures of which have been deduced on the basis of their 1 H and 13 C n.m.r. spectra, including NOE and spin decoupling experiments.

TRANSFORMATIONS OF LIGNANS, PART V. REACTIONS OF DDQ WITH A GMELINOL HYDROGENOLYSIS PRODUCT AND ITS DERIVATIVES

Hydrogenolysis of gmelinol 8 with sodium in liquid ammonia gives a triol 9, which is converted under various reaction conditions into a range of derivatives including the di- and tri-O-methyl ethers 10 and 11, a 3,4-dibenzyl-3-hydroxy-tetrahydrofuran 12, and its acetate 13. These derivatives undergo oxidative cyclisation with DDQ in acetic acid or trifluoroacetic acid to yield 1-aryltetralin, 1-arylnaphthalene, dibenzocyclooctadiene and spirodienone derivatives in reactions which provide biomimetic analogies for biogenetic transformations of lignans.

Rearrangements of the 2,6-diaryl-3,7-dioxabicyclo[3.3.0] octane skeleton, part III. Reaction of gmelinol with BF3-etherate and N,N-dimethylaniline

Abstract Reaction of gmelinol 1 with BF 3 -etherate and N,N-dimethylaniline gave a product 9 formed by rearragement of the 2,6-diaryl-3,7-dioxabicyclo[3.3.0]octane skeleton. Compound 9 on reduction with BF 3 -etherate and triethylsilane produced 15 which is epimeric with di-O-methyl cycloolivil 18 .

Transformations of lignans. Part 4: Oxidative and reductive rearrangements of dibenzocyclooctadiene and spirodienone lignans

DDQ oxidation of a dibenzocyclooctadiene derived from a 2,3-dibenzylbutane-1,2,4-triol di-O-methyl ether gives an oxygen-bridged dibenzocyclooctadienone and an oxygen-bridged spirodienone. The same products may also be obtained directly from the dibenzylbutane derivative by treatment with excess DDQ. Acid treatment of the spirodienone leads to demethylation and rearrangement to an ortho-benzoquinone. Reductive rearrangement of the spirodienone produces an oxygen-bridged dibenzocyclooctadienone. These reactions yield unique cyclohexadienone lignans and illustrate readily achieved increases in complexity starting from a simple dibenzylbutane derivative.

TRANSFORMATIONS OF LIGNANS, PART IV. ACID-CATALYSED REARRANGEMENTS OF GMELINOL WITH BF3-ETHERATE AND STUDY OF A PRODUCT WITH A UNIQUE LIGNAN SKELETON FORMED BY FURTHER OXIDATION WITH DDQ

Abstract Reaction of gmelinol 1 with BF3-etherate followed by treatment with various additives gave two isomeric products 2 and 5 formed by rearrangement of the 2,6-diaryl-3,7-dioxabicyclo[3.3.0]octane skeleton. Compounds 2 and 5 on oxidation with DDQ in trifluoroacetic acid or benzene produced enantiomers 8 and ent-8 respectively, which have a “distorted furofuran” skeleton. Compound 5 on reduction with triethylsilane and BF3-etherate gave 13, which is isomeric with di-O-methyl cycloolivil.

A general synthesis of 2,6-diaryl-3,7-dioxabicyclo[3.3.0]octane lignans applicable to the synthesis of unsymmetrical lignans

Abstract Utilising a new four carbon synthon suitable for regiospecific and stepwise anion production, unsymmetrically substituted 2,6-diaryl-3,7-dioxabicyclo-[3.3.0]octane lignans have been synthesised for the first time.

Serendipitous synthesis of diaxial 2,4-diphenyl-3,7-dioxabicyclo[3.3.0]octane

Abstract Reduction of cis-3,4-dibenzoyltetrahydrofuran with BMS, very surprisingly yields diaxial 2,4-diphenyl-3,7-dioxabicyclo[3.3.0]octane in a reaction that contrasts sharply with processes involving equilibrating benzylic carbocations which yield the diequatorial isomer.

A general synthesis of 2,6-diaryl-3,7-dioxabicyclo[3.3.0]octane lignans applicable to unsymmetrically substituted compounds

A new synthesis is described of 2,6-diaryl-3,7-dioxabicyclo[3.3.0]octanes which has been used to prepare unsymmetrically substituted lignans such as methyl piperitol, and affords the first stereoselective synthesis of equatorial–axial isomers such as methyl pluviatilol and methyl xanthoxylol. The method utilises a new four carbon synthon (6) which allows regiospecific and stepwise anion production. Two alternative routes are given from the 2,6-diaryl-4,8-dimethoxy-1-methylthio-3,7-dioxabicyclo[3.3.0]octanes (14b) and (14c) to the parent lignans. One of these procedures shows that the methylthio group is capable of acting as a highly effective stereocontrol element. The stereochemical consequences of both procedures have been analysed and can be rationalised in terms of equilibration during a Lewis acid catalysed reduction step. Thus, while one route affords selectively methyl pluviatilol (19) in high yield, the other route gives a 2:1:1 mixture of methyl piperitol, methyl pluviatilol, and methyl xanthoxylol.