Stereochemistry cyanation

The oxyanions as ligands may be classified according to (a) the structural type of the oxyanion (X02, X03, X04 or X06) (b) the coordination number of the oxyanion (1-18), i.e. the number of metal atoms to which a single oxyanion may be coordinated (c) the mode of coordination of the oxyanion, i.e. monodentate, bidentate, tridentate, etc. and (d) the number of oxyanions per metal atom, the stoichiometry p, from one to six, i.e. [M(XO ) ]. Table 1 lists the oxyanions that will be considered in this section according to their structural types, with their approximate stereochemistry and point group symmetry. The carbon-containing oxyanions will be described in Chapter 15.6, and the cyanates in Chapter 13.5, For reasons of space this review will be primarily restricted to mononuclear oxyanions. Figures 2-5 illustrate the mode of coordination of the oxyanions as a function of their coordination number 1-18. 

The corresponding /i-amino aldehydes are reduced in situ and the corresponding amino alcohols are isolated in good yield with up to >99 % ee. The Mannich reactions proceed with excellent chemoselectivity and inline formation occurs with the acceptor aldehyde at a faster rate than C-C bond-formation. Moreover, the one-pot three-component direct asymmetric cross-Mannich reaction enables aliphatic aldehydes to serve as acceptors. The absolute stereochemistry of the reaction was determined by synthesis and reveled that L-proline provides syn /i-amino aldehydes with (S) stereochemistry of the amino group. In addition, the proline-catalyzed direct asymmetric Mannich-type reaction has been connected to one-pot tandem cyanation and allylation reaction in THF and aqueous media affording functional a-amino acid derivatives [39, 42]. 

In recent years some alternative, more refined, and higher yielding processes have been developed. One method that employs milder conditions than the earlier ones and avoids the formation of the N, -oxide involves ozonization of vincadifformine (76) in 0.43 M sulfuric acid in methanol at 60°C, which gives a 74% yield of a 7 3 mixture of vincamine (286) and 16-epivincamine in a one-pot reaction (276). Here again the 16-hydroxyindoienine derivative 285 is an intermediate, since it can be isolated if the ozonization reaction is conducted at 20°C. The stereochemistry of 285 follows from its reaction with potassium cyanate in dii clohexyl-lS-crown-6 and methylene chloride, which affords the hexai cUc urethane 294. Similarly, the ozonization of tabersonine (78) at 65°C affords a 71% yield of a mixture of 14,15-didehydrovincamine (287) and its 16-epimer (276). 

As illustrated by comparing the results shown in Scheme 2.2.16 with those illustrated in Scheme 2.2.17, the stereochemistry of the initial cyanation is dependent upon the stereochemistry of the starting material. Furthermore, the formation of the opposite 3-hydroxy configurations following hydride reduction is noted. 

The relative stereochemistry of epoxides can be inverted by eqnilibration with cyanate anion ... 

Thiazolidine-2-thiones can be prepared with complete control over stereochemistry from the olefins (134). Addition of iodoiso-cyanate to the olefins in MeOH generates the known B-iodocarbamates (I35) which on treatment with potassium ethyIxanthate followed by base give the thlazolidine (136). The opposite stereoisomer (138) is formed by addition of the xanthate to the azlrldine (137) formed from base treatment of (135). ... 

Audran and colleagues described two syntheses of furanosesqui-terpene natural products employing similar synthetic pathways, culminating in the use of the Barton-McCombie deoxygenation as a key step. The ester unit in bicyclic lactones 65 and 66 is used to set stereochemistry of the C-2 center. After further elaboration, the ester stereocontrol element is then reductively cleaved and cyanated to yield the diols 67 and 68, from 65 and 66, respectively. Then, in a one-pot, two-step process, activation and double deoxygenation was performed to yield reduced compounds 69 and 70, which are the intermediates in the synthesis of (-i-)-ricciocarpin A (71) and (+)-ancistrofuran (72), respectively. 

---