682. Photochemical transformations. Part XIV. Some analogues of isophotosantonic lactone

Abstract

The previously discovered rearrangement of a-santonin to isophoto-asantonic lactone has been shown to be a general phenomenon. p-Santonin, 6-epi-ct-santonin, 6-epi-P-santonin, artemisin acetate, 6-epi-8-epiartemisin acetate, and 8-epiartemisin acetate all undergo the corresponding rearrangements. With the aid of X-ray crystallography the stereochemistry of this transformation has been elucidated. Differential hydrogenation or hydrogenolysis reactions in these compounds have been observed depending upon the quasi-equatorial or quasi-axial character of the alkyl-oxygen atom of the lactone ring.

Conversion of (−)-α-santonin into (+)-α-cyperone

“…In this connection, it is pertinent to point out that the catalytic hydrogenation of (-)-a-santonin (5) and its derivatives has been studied in some detail (8). From the results of this work, it appeared unlikely that the use of usual heterogeneous catalytic hydrogenation would effect complete selective removal of the C1-C2 double bond in the presence of the C4-C5 double bond.…”

“…Conversion of (-)-a-santonin (5), of known absolute stereochemistry (4,5), into the epimeric (-)-6-epi-a-santonin (6) was readily accomplished by a procedure similar to that described by Ishikawa (6). Treatment of compound 6 with zinc dust in glacial acetic acid -methanol (7) caused reductive cleavage of the cis-fused lactone ring and afforded a very good yield of the carboxylic acid 7.…”

“…Thus, the isolated yield of 1 reported by Howe and McQuillin (2) was approximately only 4%. We have, therefore, considered alternative methods for the preparation of (+)-a-cyperone (1) and report in this paper its synthesis from the wellknown and readily available sesquiterpene, (-)-a-santonin (5).…”

Conversion of ( + )-α-cyperone and ( – )-7-epi-α-cyperone into hydroazulenederivatives

“…Upon consideration of various synthetic path\vays which would give an entry into this system, i t becomes obvious that the photochemical rearrangement of cross-conjugated cyclohexadienones provides an excellent and efficient method for the preparation of specifically substituted hydroazulenes (4)(5)(6)(7)(8)(9). Furthermore, the stereospecific nature (10) of this rearrangement makes the reaction particularly attractive for stereoselective svnthesis, since it allows for the control of the configuration a t certain key centers of the product.…”

“…These curves were then compared with that of isophotosantonic lactone (VIII), of known absolute stereochemistry (10).…”