B-Ring Chemistry

The C-10 acetoxy group can be modified in principle in several ways. Thus it can be hydrolyzed to the hydroxyl group and this can be derivatized with any of a large number of acylating or alkylating agents. Alternatively, it can be removed reductively, it can be oxidized, or it can be replaced by other functional groups. All of these approaches have been used in the preparation of derivatives at C-10. 

Selective hydrolysis of the C-10 acetate group of taxol or baccatin III was initially a difficult reaction, which could be achieved in modest yield by treatment of taxol with zinc bromide in methanol (57). Studies with 7-(triethylsilyl)hexahydrobaccatin III showed that the C-10 acetate is hydrolyzed first, followed by the C-4 acetate, but these reactions were not readily transferred to baccatin III itself 58). It was thus a significant advance when several investigators reported methods for selective hydrolysis of the C-10 acetate group. 

A number of investigators have made C-10 acyl analogs of taxol. Thus Bourzat and his collaborators prepared various 10-(9-carbamoyl analogs of taxol such as 3.1.2.2 by treatment of 7,10-di-troctaxol (3.1.2.1) with various amines, followed by deprotection of the unreacted C-7 Troc group (77). 

Rao and his collaborators prepared a number of 10-acyl analogs from 10-deacetyltaxol by protecting the 2 and 7 positions as their chloro-acetyl derivatives 3.1.2.3, acylating at C-10 and deprotecting with thiourea to give the 10-acyl analogs 3.1.2.4 (72). 

A different route was used by Kant and his coworkers 73) and by Ojima and his collaborators 74). Both groups selectively acylated 7- 

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P2. Pannala, A. S., Chan, T. S., O brien, P. J., and Rice-Evans, C. A., Flavonoid B-ring chemistry and antioxidant activity Fast reaction knetics. Biochem. Biophys. Res. Commun. 282, 1161-1168 (2001). 

Sekher PA, Chan TS, O Brien PJ, Rice-Evans CA. Flavonoid B-ring chemistry and antioxidant activity fast reaction kinetics. Biochem Biophys Res Commun 2001 282 1161-1168. 

The biologically active oxazole compounds were synthesized by Wang et al. [34], and two types of isomers were described those with N1 pointing towards the A-ring (e.g., 54) and those with N1 positioned closest to the B-ring (e.g., 55), Scheme 10. Tosmic reagents 12 and 13 were used for this synthesis as described in Sect. 2.1.1, Scheme 1. The chemistry described... 

Laks, P.E. 1989. Chemistry of the condensed tannin B-ring. In Chemistry and Significance of Condensed Tannins (R.W. Hemingway and J.J. Karchesy, eds.) pp. 249-263. Plenum Press, New York. 

The chemical structure of vitamin D3 is closely related to its precursor, 7-dehydrocholesterol, from which it is produced by a photochemical reaction. Therefore, vitamin Do is closely related structurally to the four-ring nucleus of steroids derived from the cyclopentanoperhydro-phenanthrene ring system. No vitamin D activity is noticed until the B ring of 7-dehydrocholesterol is opened between C-9 and C-10. Thus, vitamin D3 is a 9,10-seco steroid and its carbon skeleton is numbered accordingly (Scheme I). The important aspects of its chemistry center about the 5,6,7-cis-triene structure. The formula for vitamin D3 is C27H44O and its formula weight is 384.64. 

Cassane and Miscellaneous Tricyclic Diterpenoids The Chemistry of Ring A The Chemistry of Ring b The Chemistry of Ring c... 

The rotation of phenol CLV led Dvornik and Edwards to first suggest that if the structure of atisine is based on an unrearranged diterpenoid skeleton, its A/B ring system bears a mirror image relation to that of the common diterpenes 17). Furthermore, the rotational arguments based on ajaconine chemistry reinforced this perceptive suggestion 17). 

The tautomeric benzophenone pair xerophenone A (103) and B (104) present an interesting variation in benzophenone chemistry by having a 7-membered C-ring and an oxo bridge between carbons 4 (C-ring) and 10 (B-ring). These compounds feature an oxatricyclo[4.3.1.1]undecane-7,9-dione system [79],... 

Foods of animal origin are suspected to contain some amount of COP formed by autoxidation. Cholesterol autoxidation is a well-established free radical process that involves the same chemistry that occurs for the oxidation of unsaturated lipids. Cholesterol contains one double bond at the carbon-5 position therefore, the weakest points in its structure are at the carbon-7 and carbon-4 positions. However, due to the possible influence of the hydroxyl group at carbon-3 and the tertiary carbon atom at carbon-5, the carbon-1 position is rarely attacked by molecular oxygen, and therefore the abstraction of an allylic hydrogen predominantly occurs at carbon-7 and gives rise to a series of A and B ring oxidation products. In the chain reaction,... 

Another approach, this time to the 4-demethoxy derivative 8.77, was to use the Dotz reaction to form the B ring (Scheme 8.22). The alkyne 8.74 was prepared by malonate chemistry and reacted with the tetracarbonylchromium-carbene complex 8.75. The ti -product decomplexed easily on warming to give the air-sensitive anthracene 8.76,... 

Mallion, R. B. Ring currents in coraimulene, a prototype molecule for Buckminsterfullerene. In Studies in Physical and Theoretical Chemistry, Vol. 63. Proceedings of an International Course and Conference on the Interfaces between Mathematics, Chemistry and Computer Science, Dubrovnik, Yugoslavia, June 20-25, 1988 Graovac A., Ed. Elsevier Amsterdam, 1989 pp 505-510. Mallion, R. B. Chem. Phys. Lett. 1975, 36, 170—174. 

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