The direct ultraviolet irradiation of dimethyl benzyl phosphite (1) and dimethyl p-acetylbenzyl phosphite (8) was investigated in acetonitrile, cyclohexane, and benzene. Phosphite 1 gives predominantly the photo-Arbuzov product, dimethyl benzylphosphonate (2), in 67−81% accountability yields, based of phosphite consumed, along with minor amounts of bibenzyl (20) and dimethyl phosphite (10). The quantum yield for formation of 2 in cyclohexane, φP, is 0.43. By contrast, irradiation of phosphite 8 yields only 7−13% of photo-Arbuzov phosphonate (9) but relatively large amounts of radical diffusion products:  dimethyl phosphite (10) the p-acetylbenzyl radical dimer (11); and p-acetyltoluene (12). Evidently 8, closely related to acetophenone, reacts predominantly via the triplet excited estate to generate long-lived, triplet, free-radical pairs (6 and 7a). In benzene, further products (15, 16, 17a and 17b) are identified that result from addition of the phosphinoyl radical (6) to benzene to give cyclohexadienyl radical 14, followed by combination and disproportionation reactions with radical 7a. (Total product quantum yields in benzene (Σφi) = 0.47.) In benzene, accountabilities of radical 6 from photolysis of 8 as high as 56% are encountered along with up to 92% accountabilities of p-acetylbenzyl (7a) radicals. Addition of radical scavengers PhSH, PhCH2Br, and TEMPO in the three solvents establishes the cage yield of 9 as 3 − 5%. The products of radical trapping provide further proof of the radical-pair nature of the photolysis of phosphite 8, including a 95% accountability of 6 with PhCH2Br in benzene. It is proposed that the CH2−O scission of triplet 8 must occur concertedly with partial phosphoryl (PO) bond formation. The trapping of radicals 6 and 7b from irradiation of phosphite 1 as the benzene adducts 22 and 23, analogous structurally to those (16 and 17) from phosphite 8, supports the postulation that photoisomerization of 1 to 2 proceeds via short-lived, presumably singlet, free-radical pairs.

The boron enolate of pyrone 2 undergoes asymmetric aldol reactions with aldehydes to give protected anti 1,2-diols 3. The pyrone is readily available from trans stilbene using asymmetric dihydroxylation. Yields for the aldol reaction range from 62 to 92% and the selectivities from 6:1 to >20:1 for the anti isomers. Protection and hydrogenolysis of the products can be used to remove the pyrone, giving differentially protected diol intermediates 12 that are amenable to multistep synthesis.

Geldanamycin (GA), an antitumor Hsp90 inhibitor, was made for the first time by using an oxidative demethylation reaction as the final step. A biaryldioxanone auxiliary set the anti C11-12 hydroxy-methoxy functionality and a methylglycolate auxiliary based on norephedrine was used for the syn C6-7 methoxy-urethane. p-Quinone-forming oxidants, CAN and AgO, produced an unusual aza-quinone product. Nitric acid gave GA from a trimethoxy precursor in 55% yield as a 1:10 mixture with nonnatural o-quino-GA. [structure: see text]

Recently we have reported in situ generation of electrophilic selenium (PhSe+) species via diphenyl diselenide radical cation (PhSeSePh)*+ formed by single electron transfer (SET) from PhSeSePh (1) to the excited singlet of 1,4-dicyanonaphthalene (DCN,2).* 1We report herein electron transfer (ET) from compounds possessing a carbon-selenium bond to 'DCN* leading to heterolytic C-Se bond dissociation via a radical cation intermediate with concurrent demonstration of a new, very mild, synthetically promising way of effecting deselenenylation reactions.Organoselenium compounds 5-8 (Scheme I) efficiently quench the fluorescence of 2 with a diffusion-controlled2 rate constant, ( ). Emission and absorption spectroscopic properties of mixtures of 5-8 and 2 fail to reveal ground or excited state complex formation in polar and nonpolar solvents. A SET mechanism for the fluorescence quenching of 2 by 5-8 is supported by the endoergic value for the free energy change (AGET) estimated by the Weller equation3 and the correlation plot of log AL^v s (£j/2(ox))4 of 5-8, which showed a linear relationship5 (Table I). This suggested the possibility of performing photosensitized SET initiated reaction from compounds 5-8. To test this concept, we irradiated6 78(>280 nm) a mixture of 5-8 (3 mM) with 2 (0.3 mM) in methanol, for which the details are given in Table I. Bibenzyl formation from compound 6 seems to occur by deselenenylation of PhCH2Se* (PhCH2Se* -PhCH2* + Se), which is evident from the observation of only a trace amount of bibenzyl (3-6%) compared to other products.7The above results can be rationalized with reference to Scheme I, which considers the intermediacy of an exciplex with partial charge-transfer character. A double reciprocal plot of quantum yield ( ^ ) vs 5-8 concentration ( ~ vs [Q]"1) resulted a straight line with an identical AfqETT (intercept/slope) as obtained from fluorescence measurements (Table I), which supports the above argument. Electron transfer between 5-8 and 2 results in generation of the solvent-separated ion pair (SSIP). The corresponding radical cation undergoes C-Se bond cleavage (58 kcal/mol),8 the carbonium ion escaping and the radical reacting in the cage. In the polar media R|+ is more stabilized than the radical R2Se* so that the former diffuses and the latter reacts with 7IICT Communication No. 2425.

[figure: see text] A synthesis of the left-hand portion of the ansamycin antitumor natural product geldanamycin is reported. An advanced intermediate incorporates the methoxyquinone precursor as a pentasubstituted benzene with a 10-carbon chain that contains 4 of the 6 stereocenters. The key reaction is a novel anti glycolate aldol reaction with a new diaryl-4-oxapyrone used to generate the C-11, C-12 hydroxy, methoxy functionality.