Aluminium hydride, irradiation

It was reduced with lithium aluminium hydride in tetrahydrofuran to yield the alcohol (40) whose structural assignment was derived from H NMR spectroscopic data. Irradiation of a mixture of alcohol (40) and lead... 

A photochemical preparation of a Cephalotaxus alkaloid synthon (20) has been reported (Scheme 2).10 The readily accessible maleimide (17) was iodinated with iodine and silver trifluoroacetate, in 71% yield, and the resulting compound was transformed in two steps (70% overall yield) into the methylene-pyrrolone (18) by the action of methylmagnesium iodide followed by dehydration. Irradiation of (18) afforded (19) (46% yield), which, by successive hydrogenation and reduction with lithium aluminium hydride, gave the dihydro-pyrrolo[2,l-b][3]benzazepine (20). This derivative has served as a key intermediate in the total synthesis of cephalo-taxine described previously (see Vol. 7 of these Reports). 

An improved route to 2a-hydroxycholesterol has been devised as part of the preparation of 2a-hydroxy-vitamin D3 (263 R1 = R4 = R5 = R6 = H, R2 = R3 = OH).123 Hydroxylation of the A bond of cholesta-l,5-dien-3/3-ol by means of 9-borabicyclo[3,3,l]nonane followed by reaction with alkaline hydrogen peroxide produced the 2-equatorial 2a,3a-diol in 70—80% yield. The conventional four-step sequence, acetylation, bromination, dehydrobromination, and hydrolysis, gave 2a -hydroxycholesta-5,7-dien-3/3-ol which was converted into 2a-hydroxy-vitamin D3. The isomeric 2/3-hydroxy-vitamin D3 has also been reported.124 Reaction of the 1/6,2/3-oxide obtained by peroxidation of the adduct (265) with lithium aluminium hydride results in a mixture of 2/3,3/3-dihydroxycholest-5,7-diene and its 1/3,3/3-dihydroxy-epimer in the ratio 8 1. Irradiation of the former 5,7-diene furnished the expected previtamin, which on equilibration gave 2/3-hydroxy-vitamin D3 (263 R1 = R4 = R5 = R6 = H, R2 = a-OH, R3 = OH). 

An approach to the total synthesis of cycloxobuxine and cycloprotobuxine alkaloids has been attempted starting from lanosterol.32 The pregnane derivative (38a), prepared from lanosterol,33 was transformed into the lljS-nitrite ester (38b), which was irradiated in benzene solution containing iodine. After oxidation of the resulting iodide, the 11 -oxo-19-iodo-derivati ve was transformed, through an alumina column,into 1 l-oxo-9/ , 19-cyclo-derivative (39) (cycloxobuxine skeleton). Reduction of (39) with lithium aluminium hydride gave a mixture of the triol (40) and the diol (41), the latter being oxidized to the diketone (42). 

Lai er Rings.—Humulene A -monoepoxide (250) is the major product of WClg-n-butyl-lithium reduction of humulene triepoxide. It is reduced by lithium aluminium hydride or lithium-ethylamine to the 5-alcohol (251 X = OH, Y = H). The isomeric 4-alcohol (251 X = H, Y = OH) can be prepared from humulene-bis-epoxide. All -cis-cyclododeca-l,5,9-triene reacts with chlorosulphonyl isocyanate to give a good yield of mono-adduct, which can be hydrolysed to the p-lactam (252). Reactions of iV-bromosuccinimide in the presence of water, methanol, and acetic acid, with CIS- and trans-cyclododecene have been examined. cis- and trans-l-Nitrocyclo-dodecenes are interconverted on irradiation further irradiation causes isomerization to 3-nitro-trans-cyclododecene. ... 

A mixture of cis,trans- and cis,cis-cyclo-octa-l,5-dienes in the ratio of cis,trans cis, CIS = 3 5, was formed by heating either syn- or anti-tricyclo[4,2,0,0 ]octane (125) in the gas phase or in solution. A biradical mechanism was proposed. The benzo-trans-cyclo-octenone (126 X = O) was obtained by irradiation of its cis-isomer. It was not isolated, but was reduced in situ with lithium aluminium hydride to the alcohol (126 X = H, OH), treated with methanol to give the bicyclic ether (127 R = Me), and converted into alcohol (127 R =... 

No evidence could be obtained for the formation of 2ff-benzo[i>]thiet or its valence tautomer by irradiation of 3/f-l, 2-benzodithiole l,l-dioxide. Naphtho-[l,8-6c]thiet (58) has been prepared and its 5-oxide, and S -dioxide, have been obtained by oxidation with m-chloroperoxybenzoic acid. All three compounds react with lithium aluminium hydride to give ring-opened products as the result of hydride attack on sulphur. After reduction and methylation, (58) gave the methyl sulphide (59), and the 5-oxide gave bis(a-naphthyl) disulphide (75%) together with (58) (6%) and the corresponding sulphone (4%), which was the major product (85%) from the 55-dioxide (Scheme 8). Thiets are believed to be... 

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