A novel Approach f o r t h e s y n t h e s i s of t h e BCD-ring system i n s t e r o i d s i s d e s c r i b e d . The sequence centers about t h e i n t r a m o l e c u l a r Diels-Alder r e a c t i o n of f u r a n 1 5 , which, depending on t h e r e a c t i o n c o n d i t i o n s , l e a d s predominantly t o a d d u c t 7 6 ( k i n e t i c c o n t r o l ) or 27 (thermodynamic c o n t r o l ) . The p r e c u r s o r is o b t a i n e b f r o m e n o l e t h e r 2, prepared v i a d i s s o l v e d metal r e d u c t i o n o f 5.SCHEME 2 -1 0 2 1 -chloride led to the denired enol ether 2 in 709 yield after purification by distillation. in viow of the reported reduction of the furan nucleus in 2-furancarboxylic Acid under similar conditionr. The selective reduction of the anone system in 2 is noteworthy 16 The synthesis of potential Dielr-Alder precursors Although 2 offers in principle the possibility for the diroct introduction of dienophilic side-chains we decided first to syntheoize various dienophiles with an oxygenated 11-position via the common intermediates 9 and fi (Scheme 2 ) .The latter aldehydes were obtainod vie regeneration of the enolate anion from -2 with methyllithium and regiorelective AlkylatiOn with methyl bromoacetate to give the --ketone 6 in 67% irolated yield, next to lesr than 78 of the corresponding *-isomer.The stereochemistry of the major isomer is assigned on the h e i r of the more shielded angular methyl resonance in the 'H NMR spectrum ( 6 0 . 6 8 ) compared to the minor isomer (6 1 . 3 0 ) , a result due to the location of the methyl protons of 6 in the shielding cone of the fury1 group.17 served stereoselectivity (?. 9 . 6 : l ) presumably results from the perpendicular approach of the electrophile on the 4-/E envelope conformation Of the enolate anion from the less hindered a-side which results in the least confonnational distortion" to yield the kinetic 1-/T twist conformation of cyclopentanone -6 . l g f 2o formation would yield the 1+/T conformation of the corresponding *-derivative (Scheme 3). tent with the observed small sum of vicinal coupling constants for H-3 in the enol ether 2 (6-7 Hz) , results from the detrimental A1,'-etrainZ1 in the 4+/E form. with leas eterically encumbering 3-alkyl substituents AS has been observed previously with a vinyl group (ratio 3 -6 r l for trans:cis, respectively) . 2 2 * 2 3The ob-The analogous perpendicular least-dirtortion pathway on the 4+/E con-The preferred population of the 4-/E conformation, which is consis-Following this rationale one may expect a decreased stereoselectivity
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