Review Journal of Chemistry (v.1, #2)

Ent-kaurane diterpenoids and glycosides: Isolation, properties, and chemical transformations by E. Kataev; R. N. Khaybullin; R. R. Sharipova; I. Yu. Strobykina (93-160).
This review is devoted to ent-kaurane diterpenoids and related glycosides, which represent two naturally occurring groups of compounds with interesting structures and properties. Much useful information about the biogenesis and biological activities of these compounds is presented; however, the greatest part of the review covers various aspects of the chemical transformations of native ent-kaurane diterpenoids, namely, their reactions via COOH, C=O, and OH groups and double bonds. Special attention is given to rearrangements of the carbon skeleton, primarily the Wagner-Meerwein rearrangement of the well-known kaurane diterpenoid steviol into its isomer, the well-known ent-beyerane diterpenoid isosteviol. A separate section is devoted to the chemical transformations of isosteviol, which are used to demonstrate how the well-known reactions of COOH and C=O groups, as well as CH2-C(O)R fragments, proceed in unusual ways when these moieties are attached to rigid hydrocarbon skeletons of naturally occurring compounds (or when they are parts of them). The final section considers ent-kaurane glycosides, focusing on those isolated from the plant Stevia rebaudiana, which is well known because they are 300–400 times sweeter than sucrose. The chemical transformations and, in some cases, biotransformations of these compounds (rebaudiosides) are described in detail, including the total synthesis of some rebaudiosides aimed at creating a production technology for low-calorie sweeteners. The review covers data published between 1964 and 2010, including original results obtained by the authors.
Keywords: kauranes; beyeranes; glycosides; diterpenoids; steviol; isosteviol; S. rebaudiana ; steviolbioside

This review deals with an important class of multifunctional reagents, namely, 2-halogen-N-phenacyl pyridinium salts 1 capable of forming betaine-ylides 2 (phenacylides) and thus undergoing 1,3-dipolar cycloadditions and various (cyclo)condensations involving the substitution of both α-halogen and keto-methylene groups. The chemistry of salts 1 and betaines 2 is closely related to the chemistry of oxazolo[3,2-a]pyridinium salts 3. This relationship shows itself in the interconversions of ylides 2 to salts 1 or 3 and, vice versa, of salts 1 and 3 to ylides 2 and in the similarity of chemical transformations not involving such interconversions. This similarity (necessarily involving the α-halophenacylide-oxazolium parallel) is rather unusual and can persist after the replacement of the pyridinium fragment by other heterocycles (azolium and azinium rings) or on passing from halogen to other leaving groups (RS, RO, NR2). This similarity is also manifested in nonheterocyclic open-chain salts where the potentially ylidic CH2CO group is attached to the halogen-iminium group or its functional analog with another heteroatom. The review on the synthesis and transformations of salts 1 and 3 and phenacylides 2 is divided into two parts. The present part is devoted to the synthesis of salts 1 and ylides 3 and the interconversions of 1, 2, and 3. It presents a detailed analysis of synthetic routes to salts 1 and their cycloiminium analogs containing α-halogen or a different (S, O, N) α-heteroatom that serves as a leaving group. The specific features of the synthesis of oxygen- and sulfur-containing salts and their ylides are revealed. Rare examples of the synthesis of aliphatic salts and their “phenacylides“ containing the same structural motif and capable of undergoing ring closure to oxazoles are given. Methods for the generation of phenacylides 2 and their analogs from carbenes and imines, as well as by the ring opening reaction of oxazolium salts are considered.