ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
A series of didehydro-3-O-methyl-estrones having a styrenic framework, with the ring-A-conjugated double bond in all three possible positions (8,9-didehydro- (6),9,11-didehydro- (1b), 6,7-didehydro- (9), and the 12,18-di-nor-8,9-didehydroestrone analog 11), were compared for their reactivity towards singlet oxygen. Under dye-sensitized photo-oxygenation conditions, both, products derived from ene-type reactions with the isolation of a stable hydroperoxide and a fragmentation product, were obtained from 6 (see Scheme 3), while only fragmentation took place for 1b (Scheme 1), Kinetic studies indicated that 6 is more reactive towards 1O2 than 1b (β = 9.2·10-3 mol·1-1 vs 3.3·10-2 mol·1-1, resp.). The observed reactivity, apparently, does not match with ene-type reaction and [2 + 2]cycloaddition being in competition, since the most activated substrate 6 preferentially yields ene-type products and their derivatives. Conformational analysis on the structure of 6 and 1b, both calculated by molecular-mechanic techniques (MMPMI) and determined by X-ray diffraction, show that the allylic H-atoms satisfy the orthogonality rule for ene-type reactions. The product distribution is best rationalized by applying Fukui's rule which takes into account a combination of electronic and geometric factors. Substrates 9 and 11 yielded photo-products arising from ene-type reaction with no stable primary products isolated (Scheme 4). Geometric considerations based on the calculated structures by molecular mechanics are consistent with the observed results.
Additional Material:
2 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19910740705
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