ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The stereochemical outcome of the base-catalyzed cyclization of diketones 5-8 has been investigated under protic conditions (Scheme 3). The more stable trans-fused ketols are preferentially formed in kinetically controlled aldol reactions, when the incipient angular substituent R = H (6 → 10a) or CN (7 → 11a, 8a → 12a). For R = Me (as in 5), axial attack of the side-chain enolate double bond on the ring C≡O group results in the rather selective formation of cis-9b. It is assumed that these cyclizations are controlled by relative product stabilities (product-like transition state) and steric effects. The competition between fused (e.g. 9) and bridged ketol (e.g. 13) formation in these cyclizations is discussed. The cis-fused (‘steroid’) ketols were readily equilibrated with their trans-counterparts (9b ⇄ 9a, 10b ⇄ 10a, 11b ⇄ 11a) under aprotic conditions (5 mol-% of LDA, THF, 0°), thus, allowing assessments of relative stabilities.
Additional Material:
6 Tab.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19900730607
