Ring expansion, acid catalyzed lactones

From four-membered rings An acid-catalyzed transformation has been observed in the conversion of l-[l-methylsulfinyl-l-(methylthio)alkyl]cyclo-butanol to 3-methyl-2-(methylthio)cyclopentanone [9]. - Rearrangement of a /3-lactone to a y-lactone derivative in the presence of magnesiumdibromide [10]. - A borontrifluoride catalyzed cyclobutene to cyclopentene rearrangement [11]. - Ring expansion of a [2+2] photoadduct to a five-membered ring [12]-... 

In accord with an a f/-parallel relationship between the migrating carbon (C20) and the entering nucleophile, the carbon chlorine bond in 364-Cl is found to be in the equatorial (a) configuration. On the other hand, the acid-catalyzed cyclization of methyl betulonate (367), and related diol occurs so as to form an axial carbon-oxygen bond to C19, despite the ostensibly unfavorable stereoelectronics 453, 454). It is possible, however, that this transformation is more complex than a simple ring expansion followed by lactonization. 

Transition metal catalyzed ring expansions of cyclic ethers to lactones under pressures of CO [51, 52] have been reported for tetrahydrofuran [53], oxetanes, and epoxides [54—56]. Carbonylation of epoxides is particularly important since P-lactone products are challenging synthetic targets (see Section 2.2.5). Using Co(CO)4 in combination with a Lewis acidic Al-salen counterion, the reaction of (R)-propylene oxide and CO occurs with stereochemical retention (Scheme 2.23) [57]. The mechanism is believed to involve Lewis acid activation of the epoxide followed by nucleophilic ring opening with Co(CO)4 [58]. 

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