Ring stereospecific

Modifications of Cs-symmetric metallocenes may lead to Ci-symmetric metallocenes (Fig. 8). If a methyl group is introduced at position 3 of the cyclopentadienyl ring, stereospecificity is disturbed at one of the reaction sites so that every second insertion is random a hemiisotactic polymer is produced (276, 277). If steric hindrance is greater (e.g., if a /-butyl group replaces the methyl group), stereoselectivity is inverted, and the metallocene catalyzes the production of isotactic polymers (178-180). 

RING STEREOSPECIFICITY 1 RING AROMATIZATION 2"" RING CYCLIZATION (yes or no)... 

The total synthesis of (+)-asimicin, which belongs to the family of Annonaceous acetogenins, was completed by E. Keinan and co-workers. In order to create one of the tetrahydrofuran rings stereospecifically, an intramolecular Williamson ether synthesis was performed between a secondary alcohol and a secondary mesylate using pyridine as the base. 

Jacobsen and co-workers have used similar catalysts in the enantioselective opening of epoxide rings. Stereospecific hydrolysis with a cobalt acetate chelate can be used to resolve racemic epoxides.97 Propylene oxide was opened with trimethylsilylazide in the presence of a 7 7 chromium azide chelate catalyst to produce (5)-l-azido-2-trimethyl-siloxypropane in quantitative yield with 97% ee.98 Cyclohexene oxide was opened with benzoic acid in the presence of 1 mol% cobalt chelate catalyst to give the hydroxyben-zoate in 98% yield with 77% ee.99... 

Pyridine hydrobromide perbromide 3-Amino-4-bromotetrahydrothiophene from 4-yinyltbiazolidine ring Stereospecific rearrangement... 

Sodium hydride tetra-n-butylammonium iodide 2-Azetidinone ring Stereospecific ring closure... 

Ketones from alkoxycyclopropane ring Stereospecific introduction... 

The stereospecific synthesis of an A ring synthon of la-hydroxy vitamin D has been carried out. The ( )-allcene is cyclized to give the (E -c.xo-diene 155, and the (Z)-allcene affords the (Z)-e,xo-diene 156 stereospecifically[125,126]. These results can be understood by the cis addition and syn elimination mechanism. 

A technique based on ozonation, in contrast, provides information on the stmcture of the lignin side chain by degrading the aromatic rings (33). Thus the side chain of the dominant stmcture ia all native lignins, the arylglycerol—P-aryl ether moiety, can be obtained ia the form of erythronic and threonic acids. Ozonation proves to be an elegant method for determination of the stereospecificity ia lignin. 

Some of the newer compounds may contain both saturated and unsaturated rings, heteroatoms such as oxygen, nitrogen, or sulfur, and halogen substituents. Others, such as synthetic pyrethroids, may have one or more chiral centers, often needing stereospecific methods of synthesis or resolution of isomers (42). Table 4 Hsts examples of some more complex compounds. Stmctures are shown ia Eigure 1 (25). 

Potassium Amides. The strong, extremely soluble, stable, and nonnucleophilic potassium amide base (42), potassium hexamethyldisilazane [40949-94-8] (KHMDS), KN [Si(CH2]2, pX = 28, has been developed and commercialized. KHMDS, ideal for regio/stereospecific deprotonation and enolization reactions for less acidic compounds, is available in both THF and toluene solutions. It has demonstrated benefits for reactions involving kinetic enolates (43), alkylation and acylation (44), Wittig reaction (45), epoxidation (46), Ireland-Claison rearrangement (47,48), isomerization (49,50), Darzen reaction (51), Dieckmann condensation (52), cyclization (53), chain and ring expansion (54,55), and elimination (56). 

Synthesis of 2-thioxopenams (76) has also been realized usiag a [sulfur, C-5] ring closure (117). Cyclization of 4-(3)-chloroazetidinone [88816-44-8] (77, R = R = SCOCH ), C22H22ClN20gS2, usiag imidazole ia aqueous dioxane proceeded stereospecifically to give the 5-(R)-thioxopenam [83362-57-6]... 

Ring contraction of 2-thiocephems has also been examined as a route to penems. Desulfurization of (82, n = 0) using triphenylphosphine gave mixtures of 5(R)- and 5(5)-penems (121). The stereochemical problem was neatiy overcome by regioselective oxidation to the thiosulfonate (82, n = 2) which underwent stereospecific thermal extmsion of sulfur dioxide (122) to give the S(R)-penem (83). 

The converse situation in which ring closure is initiated by the attack of a carbon-based radical on the heteroatom has been employed only infrequently (Scheme 18c) (66JA4096). The example in Scheme 18d begins with an intramolecular carbene attack on sulfur followed by rearrangement (75BCJ1490). The formation of pyrrolidines by intramolecular attack of an amino radical on a carbon-carbon double bond is exemplified in Scheme 19. In the third example, where cyclization is catalyzed by a metal ion (Ti, Cu, Fe, Co " ), the stereospecificity of the reaction depends upon the choice of metal ion. 

The parallel ability of oxiranes to undergo ring opening to carbonyl ylides was first noted in the case of tetracyanooxirane (68T2551), but such reactions have not been widely exploited. The addition to alkenes, leading to formation of tetrahydrofurans, is stereospecific (Scheme... 

---