By Gordon W. Gribble and Thomas L. Gilchrist (Eds.)
This quantity of development in Heterocyclic Chemistry (PHC) is the fourteenth annual evaluation of the literature, overlaying the paintings released on vital heterocyclic ring structures in the course of 2001. during this quantity there are really expert reports. the 1st, by means of Jan Bergman and Tomasz Janosik, covers their paintings on sulfur-containing indoles. the second one, through David Knight, discusses five- endo -trig iodocyclisations. the following chapters, prepared by way of expanding heterocycle ring dimension, evaluation contemporary advances within the box of heterocyclic chemistry with emphasis on synthesis and reactions.
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Extra info for A Critical Review of the 2001 Literature Preceded by Two Chapter on Current Heterocyclic Topics
The very high levels of stereoselection observed in iodocyclizations of (E)-homoallylic alcohols are also consistent with a relatively unfavoured cyclization which will, necessarily, be much more demanding in terms of transition state geometry. In contrast, highly favoured 5-exo-trig halocyclizations are often relatively non-stereoselcctive <87T3309; 93TA1711>. A similar D. W. g. 10, 47] cyclize easily and are less prone to competing attack on the iodonium species by water, to give iodohydrins, as the hydroxyl function can hydrogen bond with the ester carbonyl and hence is activated with respect to O-H bond cleavage.
C02M e 86 29 HO. TIPSO. . . . , I C02Me , I(D I 87 @ NMe3 88 89 Further proof of the intermediacy of the iodohydrins 85 in the formation of the hydroxytetrahydrofurans 80 came from two sources. Firstly, treatment with potassium carbonate led to formation of the corresponding epoxides. Secondly, by providing a second alkene function, suitably positioned to trap the iodohydrin hydroxyl by a 6-exo-trig iodocyclization, we have been able to intercept these species and hence define a new approach to substituted pyrans.
As the only difference in this type of substrate is the ester function, we suspect that this is involved in the iodohydrin formation, and thus provides an alternative pathway to the evidently disfavoured, direct 5-endo iodocyclization. An explanation is based on conformation 81, closely related to that deduced for the 6-exo iodolactonizations described above. Cyclization provides the stabilized carbocation 82 which is trapped by water to give the ortho ester 83. Regeneration of the ester function and ring opening then provides a single diastereomer of iodohydrin 84 which, when redrawn in conformation 85, shows how the final product 80 is formed.
A Critical Review of the 2001 Literature Preceded by Two Chapter on Current Heterocyclic Topics by Gordon W. Gribble and Thomas L. Gilchrist (Eds.)