Ring cleavage stereospecific

The singlet excited state photochemistry of cyclopropenes is dominated by ring cleavage reactions (Section IV.B.2), but in the triplet manifold [2 -I- 2] cycloaddition is the major pathway. Because of inefficient intersystem crossing the first excited triplet state of a cyclopropene is best populated by sensitization. Despite the added energy involved with these species there is no effective pathway available for ring cleavage in the absence of added olefin stereospecific dimerization to a cis-transoid-cis-tricyclohexane, e.g. 145, occurs. The smallest of the C(3) substituents is endo in the product ". A stepwise... 

The use of relatively rigid, chiral alkenes, e.g. cycloalk-2-enones, leads to a significant stereochemical induction, i.e. face selectivity of the attack. Interestingly, 4-methylcyclopent-2-enone stereospecifically reacts to provide ejco,c -3-methyl-4-methylenehexahydropentalen-l(2//)-one (22), " a product of formal proximal ring cleavage, in the presence of a phosphane-modified nickel catalyst. 

Photochemical elimination of N2 from bicyclic azo compounds produces cyclic cu-l,/t-biradicals followed by stereospecific ring cleavage or cycliza-... 

It is widely recognized that Et Zn-H O system is one of the most active catalysts for the stereospecific polymerization of oxiranes. A variety of chemical species are formed in the following way rapid formation of ethylzinc hydroxide, its aggregation, and elimination of ethane to form zinc oxide structure. The maximum catalyst activity was achieved when the mole ratio of zinc to water was one to one, where the predominant formation of a species, Et(ZnO) H, (III), was observed. If we use less amount of water, another species, Et(ZnO) ZnEt, (IV), was also produced concurrently. Contrary to the anionic nature of the former species (III), the latter species (IV) exhibited a cationic nature. For instance, more than 957o of ring cleavage of methyloxirane takes place at O-CH bond with species (III), while the cleavage at 0-CH bond also takes place concurrently with species (IV). 

Some efforts have been directed towards stereospecific synthesis of the hitherto unknown optically active ecgonine and cocaine epimers. Eogoninol (XX) (45, 54) was converted into the four-membered ring ether (XXXIII) (47, 48), and this, in turn, was submitted to ring cleavage with inversion at C3 to give 2j8-hydroxymethyl-3o-tropanol... 

Trost and co-workers have reported the coupling of 7-methyloct-Z-2-en-l-yl bromide with the dianion of methyl acetoacetate to give (168) (R=Hi) (186). The diazoderivative (168) (R=Na) was thermolyzed in the presence of copper powder to give the cyclopropane (169). Ring cleavage of the cyclopropyl ring using lithiiun dimethyl cuprate resulted in stereospecific formation of the cholesterol side chain where the relative orientation of the ultimate C-17 and C-20 positions in (170) is the same as that of the natural product. 

Natta, G Dall Asta, G Bassi, I. W. CareUa, G. Stereospecific ring cleavage homopolymerization of cycloolefins and structural examination of the resulting homologous series of linear crystalline fran -polyaUcenamers. Makromol. Chem. 1966, 91, 87-106. 

The conclusion was that with some initiator systems, abnormal ring cleavage can occur in the polymerization of propylene oxide. However, with some initiator systems, notably dialkylzinc/water or some alcohols, the initiation sites are strongly stereoselective, producing stereospecific polymer of narrow molecular weight distribution. The selectivity appears to occur in the monomer coordination step at the active initiator site. 

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