Trans-fused hydrindane

The driving force for ring expansion of 17-hydroxy-20-keto steroids probably comes from relief of strain accompanying conversion of trans-fused hydrindanes to tra/75-fused decalins. The greater susceptibility to rearrangement of 17/ -hydroxy-20-ketones as compared to the 17a-hydroxy-20-... 

Rhodium(I)-catalysed 2 + 2 + 2-cycloadditions of ene-allene (96) with allene (97) yielded trans-fused hydrindanes (98) and decalins with high regio- and stereo-selectivities (Scheme 31). A Ni-phosphine complex catalysed the 2 + 2 + 2- (g) cycloaddition of diynes with substituted ketenes to produce 2,4-cyclohexadienones in high yields. No decarbonylation products were observed. The chiral A-heterocyclic carbene-catalysed enantioselective 2 + 2 + 2-cycloaddition of ketenes with CS2 produced l,3-oxatian-6-ones in good yields and with high enantioselectivities. 2" ... 

The application of nonenzymic biogenetic-like olefinic cyclizations for the stereospecific construction of polycyclic systems, developed by W. S. Johnson and his school, represents an exciting new approach to steroid total synthesis. The synthesis of progesterone, outlined below, is based on the finding that an appositely placed acetylenic bond participates in polyolefinic cyclizations directly to produce the C/D trans-fused hydrindane system... 

Because of the possibility of nitrogen inversion, cis- and trans-fused indolizidine have no separate chemical identity. However, conformational analysis of the indolizidine and quinolizidine systems suggests that the trans is significantly more stable than the cis conformation. A value for the energy difference of 2.4 kcal mole "1 has been obtained, compared with ca. 0.3 kcal mole-1 for hydrindane.276,277... 

Due to a deformation of the cyclohexane ring, c/5 -hydrindane (the numbers in [ ] refer to carbon atoms between the bridgehead carbons) is only slightly less stable than the trans isomer ( 1 kcal/mol). 8-Methylhydrindane is of interest since it occurs as part structure of the C/D rings of steroids (as a trans fused ring system). Introduction of an angular CH3 group makes the cis isomer more stable than the trans because the cis isomer has fewer 1,3-diaxial interactions. 

The important trans-fused, angularly methylated hydrindane system has been synthesized by means of an internal Michael reaction. Thus, treatment of the enone (85) with zirconium isopropoxide gave, after dehydration and hydrogenation of the ketol (86), a 40 1 mixture of trans- to cw-hydrindanes (87). ... 

The ring-chain tautomerism of tetrahydro-l,3-oxazines is very sensitive to the stability differences, the substituents and the ring-fusion effect (Section IV,A). It also reveals a considerable stability difference in favor of the cis isomers. In the reactions of the cis- and trans-2-amino-l-cyclohexanols, as compared with the hydrindane analog systems, where the heteroatoms form an oxazolidine ring cis- or tra 5-fused with cyclohexane, the corresponding stability differences were again found to be in favor of the cis isomer (93JOC1967). 

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