Thallous cyclopentadienides

Enantioselective total syntheses of (-)-6-epitrehazolin and (+)-trehazolin were achieved by the synthesis of 275, which began with an asymmetric heterocycloaddition between [(benzyloxy)methyl]cyclopentadiene (263),108 prepared from thallous cyclopentadienide, and the acylnitroso compound arising from in situ oxidation of (,S )-mandelohydroxamic acid (264) with tetrabutylammonium periodate. Cycloaddition led to a mixture of 265 and its diastereomer (Scheme 35).109 The inseparable mixture was reduced to afford cyclopentenes 266 and 268 in 40% and 11 % overall yields, respectively, from thallous cyclopentadienide. Catalytic osmylation of 266 favored syn addition, while the osmylation of diacetate 267 was more selective and nearly quantitative, affording, after acetylation, compounds 270 and 269 in >5 1 ratio. 

A stirred suspension of thallous cyclopentadienide (prepared from cyclopenta-diene, thallous sulfate, and KOH under No, Y >91%) in dry ether treated gradually at -20° under N2 with monochloromethyl ether, after 5 hrs. the crude intermediate isolated, allowed to react 18 hrs. at 0° with 4 equivalents 2-dilor-acrylonitrile and cupric fluoroborate, the resulting crude bicyclic chloronitriie dissolved in dimethyl sulfoxide, and treated 1.5 hrs. at 30-35° with aq. KOH product. Overall Y 50-55%. E. J. Corey, U. Koelliker, and J. Neuffer, Am. Soc. 93, 1489 (1971) f. e. s. ibid. 93, 1491 electrophilic substitution of cyclopentadienide anions, review, s. M. I. Rybinskaya and L. M. Korneva, Russ. Chem. Rev. 40, 247 (1971) (Eng. transl.). 

The tetrahydrofuran complex of dicyclopentadienyl samarium is made by reduction of tricyclopentadienyl samarium with KCjgHg in naphthalene in the presence of tetrahydrofuran and was isolated as a purple pyrophoric product (Watt and Gillow, 1969). Ytterbium also reacts with thallous cyclopentadienide in dimethoxyethane in the presence of a little metallic mercury with formation of Cp2Yb DME (Deacon et al., 1982a). 

E. J. Corey has recently outlined a number of modifications of his 16-step synthetic route reported earller . Cor and his associates found that thallous cyclopentadienide W offered several advantages over... 

The use of thallous cyclopentadienide offers several advantages over the commonly used alkali metal salts, E. J. Corey, U. Koelliker and J. Neuffer, J. Amer. Chem. Soc., 93, 1489 (1971). 

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