Hydrogen bonding spirocyclic

A similar high diastereoselectivity is obtained in the catalytic hydrogenation of spirocyclic ketone 7. Delivery of hydrogen proceeds exclusively from the face of the double bond opposite the isopropyl group19. 

In some cases, product distribution is influenced by the solvent used. As observed in efforts toward the synthesis of taxane diterpenoids, intermediate 48 when left standing in methanol for three days was completely converted to 49. On the other hand, in the aprotic solvent CDCI3, 48 was present in an 88 12 ratio with 49. These results are rationalized as follows. The strain of the bridgehead double bond in 48 causes 49 to be favored thermodynamically. In the presence of MeOH, where intermolecular hydrogen bonding is predominant, spirocycle 49 is... 

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

In all the following examples, the targeted double bonds were activated by suitable substituents to increase the efficacy of the desired cyclization mode. For the total synthesis of acutumine (26), an activated a,p-unsaturated ketone 27 was chosen as precursor (Scheme 10) [74, 75], Aryl radical additions to this type of alkenes are known to proceed about ten times faster than to comparable allylic alcohols. In a radical-polar crossover reaction, the spirocyclic product 28 was obtained in the presence of triethylaluminum as promoter and an oxaziridine as hydroxylating agent. The fact that even the efficient hydrogen donor tetrahydrofuran could be used as solvent nicely demonstrates the high efficacy of the cyclization step. 

Herrera et al. [97,98] have reported the usage of Norrish-Yang photocyclization for the synthesis of new spirocyclic monosaccharide derivatives of types 243 and 244 via a hydrogen atom transfer (HAT) reaction promoted by a 1,2-diketone 241, in its excited state, followed by C-C tetrasubstituted bond formation in a diastereo-selective manner (Fig. 8.59). Of special interest is the study of the tendency to inversion at C5 (for examples of epimerization of anomeric and pseudoanomeric radicals, see [99, 100]), probably triggered by conformational changes that the 1,4-diradical intermediate 242 undergoes in its triplet state, within its lifetime (for discussions of the lifetime of diradicals in solution, see [101, 102]) before the intersystem crossing (ISC) occurs. 

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