Norio Maeda

Photo-induced transformations. Part 51. Photo- and thermally-induced rearrangements of hypoiodites of steroidal homoallyl alcohols. The formation of some oxygen heterocycles via photo- and thermally-induced rearrangements of 3-hydroxy-Δ5-steroid hypoiodites in the presence of mercury(II) oxide and iodine

Hypoiodites of cholesterol and epicholesterol in benzene containing mercury(II) oxide and iodine underwent photo-induced rearrangement to give 3α,5-epoxy-6β- and -6α-iodo-A-homo-4-oxa-5α-cholestanes (3) and (4), together with 3-formyloxy-2-iodo-A-nor-2,3-secocholest-5-ene (2). Stereochemistry of the epoxide (3) was established by an X-ray crystallographic analysis. When the reaction of cholesterol hypoiodite was induced thermally at 55–60 °C, only the 6β-isomer, accompanied by cholest-5-en-3α-yl A-homo-4-oxacholest-5-en-3α-yl ether (20) and A-homo-4-oxacholest-5-en-3α-ol (21), is formed. Catalytic hydrogenolysis of iodo-epoxide (3) or (4) gave 3α,5-epoxy-A-homo-4-oxa-5β-cholestane (16) which was transformed into 2-acetyl-5-acetoxymethyl-4-oxa-5β-cholest-2-ene (29) upon treatment with boron trifluoride–ether–acetic anhydride in benzene.In contrast, the newly synthesized hypoiodite of 3α,4,4-trimethylcholest-5-en-3β-ol, in benzene containing mercury(II) oxide and iodine, gave 2-acetyl-3-oxacholest-5-enes (26) and (27) together with 3α,5α-epoxy-A-homo-4-oxasteroids (24) and (25) on irradiation or thermolysis. The formation of the products (26) and (27) indicates the intervention of a common oxyl radical (E) in the rearrangements of 3-hydroxy-Δ5-steroid hypoiodites to products (3), (4), (24), (25), (26), and (27). The pathways of the rearrangements and the stereoselectivity of the reactions are discussed.

Photoinduced transformations. Part XXXXI. Photoinduced rearrangement of (22S,25S)-N-acetyl-5α-veratra-8,13(17)-dienine-3β,11β,23β-triol 3,23-diacetate 11-nitrite to two spiro-isoxazolines

Photolysis of the title nitrite (9) in toluene with Pyrex-filtered light or monochromatic light (362 ± 4 nm) afforded two isomeric spiro-isoxazolines, (10) and (11), designated photo-TVDTN and isophoto-TVDTN. Only the former was obtained pure and its structure was deduced from spectral evidence. The structure of the latter, obtained only as a mixture with photo-TVDTN, was inferred from spectra. The present results, in conjunction with those in the preceding paper, indicate that the presence of absence of a 5,6-double bond has little effect on the formation of these isoxazolines.

Photoinduced transformations. Part XXX. Photorearrangement of (22S,25S)-N-acetylveratra-5,8,13(17)-trienine-3β,11β,23β-triol 3,23-diacetate 11-nitrite, a fused cyclopentenyl nitrite, to two spiroisoxazolines

Irradiation of the title nitrite (3) in protic or aprotic solvents with Pyrex-filtered light induced an extensive rearrangement to afford two products with novel isoxazoline structures, designated photo-TVTTN and isophoto-TVTTN, in good yields. The structure and absolute configuration of photo-TVTTN (4) were determined by chemical and spectroscopic studies and by an X-ray crystallographic analysis of an iodine-containing derivative (14). The structure and absolute configuration of isophoto-TVTTN (5) were deduced by spectroscopic studies.A photoreaction carried out with monochromatic light (362 ± 4 nm) confirmed that light was only involved in homolysis of the O–NO bond, and that photo- and isophoto-TVTTN were formed via an unstable hydroxyimino-aldehyde (19), which was isolated. The hydroxyimino-aldehyde was transformed smoothly into photo- and isophoto-TVTTN on silica gel during t.l.c. Whereas the silica gel-induced transformation was non-stereoselective, leading to both photo-(4) and isophoto-TVTTN (5), the transformation of the hydroxyimino-aldehyde with acetic acid or [2H]acid was stereoselective, giving mainly isophoto-TVTTN (5) or [9β-2H]isophoto-TVTTN (21), respectively. Deuterium is incorporated into the TVTTNs in the course of intramolecular Michael reaction catalysed by acetic [2H]acid. This unusually ready intramolecular reaction seems to be the first reported example of a Michael addition of an oxime hydroxy-group to an αβ-unsaturated carbonyl system.

Photoinduced transformations. Part 60. Photoinduced rearrangements of cholesteryl nitrites with monochromatic light

Photolysis of cholesteryl nitrite in benzene with monochromatic light (λ 389 nm) gave (E)-4′-azodi-(3,4-secocholest-5-en-3-one)NN′-dioxide (9) which was transformed into N-hydroxy-4-aza-4a-homocholest-5-en 3-one (10) and 3,3-bisisopropoxy-3,4-secocholest-5-en-4-one oxime (20) in isopropyl alcohol under reflux. Photolysis of the dimer (9) with Pyrex-filtered light gave 3,4-secocholest-5-ene-3,4-dione 4-oxime(25). In contrast, irradiation of 4,4-dimethylcholesteryl nitrite (2) in benzene with monochromatic light gave a good yield of 4a,4a-dimethyl-4-aza-4a-homocholest-5-en-3-one (11) without any accompanying nitroso-dimers. These results as evidence that hydroxamic acids [e.g.(10)] obtained from the photolysis of nitrites are formed by thermal cyclization of nitrosointermediates [e.g., (7)].Although irradiation of the nitrite (2) with Pyrex-filtered light gave a result similar to that with monochromatic light, low yields of the Z-isomer (26) of the nitroso-dimer (9) and 3β(4-oxa-4a-homocholest-5-en-3α-yloxy)cholest-5-ene (27) were isolated from a mixture obtained from the photolysis of cholesteryl nitrite (1) with Pyrexfiltered light. Some notable features in the present results are discussed.