DEHYDROBROMINATION Subject

The formation of furans from pyrylium salts under oxidative conditions has been mentioned in Section 2.23.2.2. Ring opening and recyclization of 4-chloromethyl-2,6-dimethyI-pyrylium perchlorate (83) by aqueous alkali and DMF leads to 2-methylfuran-4-ylacetone (84) (77CHE918). When 2,3,4,5-tetraphenylpyrylium perbromide is hydrolyzed to the dienone and then subjected to bromination-dehydrobromination, 2-benzoyl-3,4,5-triphenylfuran (85) is formed. 

Fig. (2). The cyclization of enone (9), gives origin of two Cyclized products (10) and (11). Ketone (10), Ketone (10) is converted to the saturated ketone (14)under standard organic reactions.Bromination and dehydrobromination of ketone (14) yields the a,P-unsaturated ketone (IS), which on subjection to catalytic hydrogenation affords (16) and this on reduction, produces alcohol (17). The compound (13) yields (18) by standard reactions that are used for the transformation of (12) to (16). Reduction with metal hydride followed by oxidation affords ketone (11), which is converted to alcohol (17)...

Some BS-analogues with additional double bond in the A-cycle were synthesized using the a-cis-hydroxylation method developed by us earlier (13-16). As shown in Figure 4, the oxidation of 2 with osmium tetroxide followed by bromination and dehydrobromination led to dienoketone 13 which was subjected to hydroxylation under the action of iodine and silver acetate in aqueous acetic acid to give the unsaturated BS analogue 14. 

Scheme 3 Olefin (29) prepared from dienone (26) by standard organic reactions was converted to bromohydrin derivative (30), which on subjection to photolysis and dehydrohalogenation yielded ketal (32), whose conversion to a,P unsaturated ketone (34) was accomplished by hydrolysis, bromination and dehydrobromination respectively. The cyanation of (34) followed by hydrolysis and esterification produced ester (35), which on Clemmensen reduction and oxidation afforded lactone (37). Reduction of (37) followed by oxidation and esterification gave ketoester (38), which was converted to (39).

Methyldihydrotestosterone on being subjected to bromination followed by dehydrobromination gives rise to the formation of 1,2-dehydro derivative, which upon ozonization and hydrolysis yields an aldehyde-acid (1). The resulting acid (1) on reduction produces the corresponding hydroxy-acid (11) which when lactonized produces the desired compound, oxandrolone. 

Dehydration of the optically active relay (178) derived from enmein (62) gave a 1 2 mixture of 5,6-ene and 6,7-ene. Separation could be achieved by means of the ethylene acetal (187), whose ozonolysis product was subjected to successive Jones oxidation, methylation, Wittig reaction, and treatment with dilute hydrochloric acid to afford the 3-on-16-ol derivative (188). Bromination of (188) followed by dehydrobromination and subsequent dehydration afforded (189). The purified compound (189), after conversion into the acetal, was hydrolyzed to carboxylic acid (190), which was transformed into the desired lactone (191) by treatment with boron trifluoride etherate. The reaction produced a single product uncontaminated by the C-1 epimer, because of easy formation of a favored transition state which satisfied the stereoelectronic requirements. 

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