Presently, the preparation of pyridyldiphenylphosphine ligands is via the treatment of lithiopyridines with an appropriate halophosphine. In order to circumvent the major drawbacks of that procedure, i.e., low yields and the formation of unwanted pyridine side products, lithium diphenylphosphide has herein been shown to react smoothly with halopyridines to generate pyridyldiphenylphosphines. The general procedures for the synthesis of both the pyridylphosphines and the corresponding P-»0 have been described.

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As summarized in Table I, if R1 2 = aryl, spirocyclopentenone 7 was isolated as the only product (entries a-d). However, if R2 aryl, the two products 7 and 8 were isolated in almost equal amounts (entries f and g). These spiro ketones 7 and 8 can be separated by TLC [silica gel plates, with ethyl acetate/hexane (10:1) as the eluent] and are easily distinguished by their spectroscopic data.10The cyclization of 6e gave only a minor amount of the required 7e (the precursor of methylenomycin B), while the major product from the reaction was identified as the ^-substituted ethylcyclopentenone 10 [mp 84-85 °C (from CC14)], obtained in a ratio of 1:6 7e/10 (71%).n The formation of these two products is seen as apparently arising from two types of allyl anion intermediates, 9a and 9b. The other possibility, the cyclohexanone 11, which could result from the cyclization of anion 9c, was not detected (Scheme II).A modified approach was then used to prepare compound 7e. Thus when 7f or 8f was treated with 1 equiv of LDA in THF/HMPA (10:1) solution followed by addition of methyl iodide, the desired product 7e was obtained in excellent yield.12 Vacuum pyrolysis of spirocyclopentenones 7a-f and 10 at 400-450 °C (0.05 mm)13 afforded the corresponding -methylenecyclopentenones 12 in nearly quantitative ZSiUO 12 yields.10 The liquid methylenecyclopentenones 12e,f,h (R1 = Et, R2 = H) could be further vacuum distilled if needed. However, this was unnecessary since crude pyrolysates (8) Prempree, P.; Siwapinyoyos, T.; Thebtaranonth, C.; Thebtaranonth, Y. Tetrahedron Lett. 1980Lett. , 1169 It was found that the solvent ystem THF/TMEDA (4:1) gave different results from those of THF/HMPA (10:1; cf. ref 8). For example, 1973,10, 343.

A two-step reaction sequence to homoallylic nitro compounds from allylic alcohols is presented. Ethoxy carbonylation of the alcohols with ethyl chloroformate provides the corresponding allylic ethyl carbonates in high yields. Exposure of these substrates to catalytic palladium(0) in CH(3)NO(2) initiates a reaction sequence, ionization-decarboxylation-nitromethylation, that culminates with the formation of nitroalkenes. The regio- and stereochemical outcomes of the nitromethyl allylation reaction can be explained by the behavior of the transient pi-allylpalladium complexes. This methodology serves as a centerpiece for the synthesis of an important carbocyclic nucleoside intermediate.

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