Oxazolidine ring systems

Conversions of the Oxazolidine Rings in C2o Diterpenoid Alkaloids.—Several studies of the formation and opening of the oxazolidine ring system in the C20 diterpenoid alkaloids have been reported.33-35... 

Recently, Mody and Pelletier reported (198) a simple and efficient method for the degradation of the oxazolidine ring system of C20-diterpenoid alkaloids. This method involved a one-pot reaction and proceeds with almost quantitative yield. An earlier reported (199) method for this degradation involves four steps and proceeds with erratic yields. Treatment of ovatine... 

Oxazolidines are an important class of compounds commonly prepared by reaction of an amino alcohol with a carbonyl compound. The reaction of an imine with an epoxide has recently been shown to efficiently provide the oxazolidine ring system <07SL646>. The reaction is catalyzed by Yb(OTf)3 and generally provides the product oxazolidine in good yield albeit with poor ( 2 1) diastereoselectivity. 

Staphisagnine, C44H62N2O3 (resin), and staphisagrine, C43H60N2O2, m.p. 229— 231 °C, each contain an oxazolidine ring system reminiscent of atisine (108) in one unit of the dimer they differ only in the methoxy-group at C-13 in staphisagnine. 

Epimerization and Isomerization of the Oxazolidine Ring System in the C20 Diterpenoid Alkaloids A -Ray and N.M.R. Studies.—The nature of the oxazolidine ring system in the atisine- and veatchine-type alkaloids has received considerable attention during the past year. (c/. this Report, 1978, Vol. 8, p. 238). 

Degradation of the Oxazolidine Ring of C20 Diterpenoid Alkaloids.—A simple and efficient method for the degradation of the oxazolidine ring system has been developed. Treatment of veatchine (67) with acetic anhydride in pyridine, followed by complete removal of the excess pyridine and acetic anhydride, gave the diacetate (108). Without purification, this compound was refluxed in chloro-... 

Synthesis of Furan, Tetrahydrofuran, Dioxolane and Oxazolidine Ring Systems... 

These examples and those in Scheme 2.6 illustrate the key variables that determine the stereochemical outcome of aldol addition reactions using chiral auxiliaries. The first element that has to be taken into account is the configuration of the ring system that is used to establish steric differentiation. Then the nature of the TS, whether it is acyclic, cyclic, or chelated must be considered. Generally for boron enolates, reaction proceeds through a cyclic but nonchelated TS. With boron enolates, excess Lewis acid can favor an acyclic TS by coordination with the carbonyl electrophile. Titanium enolates appear to be somewhat variable but can be shifted to chelated TSs by use of excess reagent and by auxiliaries such as oxazolidine-2-thiones that enhance the tendency to chelation. Ultimately, all of the factors play a role in determining which TS is favored. 

Most of the reactions involving nucleophilic attack at a carbon atom of the ring result in cleavage of the five-membered ring system (which is in most cases either an oxazolidine or an oxazolidinone). Basic hydrolysis of... 

The ring-chain tautomerism of tetrahydro-l,3-oxazines is very sensitive to the stability differences, the substituents and the ring-fusion effect (Section IV,A). It also reveals a considerable stability difference in favor of the cis isomers. In the reactions of the cis- and trans-2-amino-l-cyclohexanols, as compared with the hydrindane analog systems, where the heteroatoms form an oxazolidine ring cis- or tra 5-fused with cyclohexane, the corresponding stability differences were again found to be in favor of the cis isomer (93JOC1967). 

A rare example (136) of the 1,2,4-oxathiazolidine ring system rearranges to the oxazolidine (137) with a half life of 14 h in CDC13 at 52 °C, cleavage of the S—O bond leading either to an intermediate diradical or to a dipolar ion (78AG(E)455). 

Initial addition reaction of methyloxirane to 4-methyl-l-oxa-4-azaspiro[4.5]decane 164 is followed by ring expansion under thermal conditions. O-Nucleophilic attack is directed on quaternary carbon of the spiro system rather than on C-4 carbon of the oxazolidine ring, and 8,10-dimethyl-7,13-dioxa-10-azaspiro[5.7]tridecane 165 is a major product of the transformation (Scheme 42 <1995IZV1838>). 

Cycloneosamandione did not contain the oxazolidine ring, but rather a tautomeric aminocarbonyl carbinolamine system (34). Two structures were proposed, one a carbinolamine formed with a C-19 aldehyde, the other a carbinolamine formed with a 6-keto group. Ultimately, the cyclone-... 

On the other hand, by bond formation with amino and hydroxy groups of ephedrine, ephedrine can be converted into chiral ring systems such as imidazolidinones, - oxazepinediones, and oxazolidines. - Diastereoselective reactions of derivatives of these chiral ring systems afford compounds with high de. The relatively rigid conformation of these ring systems is one of the reasons for high diastereoselectivities. 

A second alkaloid without the oxazolidine ring, but with the carbino-lamine system is cycloneosamandaridine (XXVII). The IR-spectrum evidences a five-membered lactone ring (band at 1780 cm i) as well as the carbinolamine group (bands at 1050/1180 cm i). The IR-spectrum of the X-acetyl compound no longer shows these hands at 1050-1180 cm i, hut an additional carbonyl frequency at 1735 cm i for an aldehyde group. The mass spectrum confirms the molecular formula as well as structure XXVII(id). 

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