Formation of a Four‐Membered Ring: Oxetanes

Abstract

Oxetanes, four-membered cyclic ethers, have a ring strain energy (SE) of approximately 110 kJ mol À1 [1] and polar properties of the CO bonds (Figure 7.1). Thus, similar to the synthetic utility of oxiranes (epoxides, SE ¼ 114 kJ mol À1 ), the ringopening reaction of oxetanes, accompanying bond-formation reactions, would be very useful for synthetic purposes [2]. Since the oxetane ring is an important structural component of biologically active compounds, such as merrilactone A [3], thromboxane A2 [4], oxetanocin [5], oxetin [6], taxane alkaloids [7], and laureacetal-B [8], efficient and selective methods to synthesize the strained structure are currently active areas of research. Moreover, oxetane-ring-containing compounds are important industrial curing agents [9]. As a consequence, there are today over 2900 patents which include the term oxetane as a key word.All of these findings clearly indicate that the demand for synthetic oxetanes is high. There are basically three methods for preparing oxetanes:1. The intramolecular nucleophilic substitution reaction. 2. The ring-expansion reaction of epoxides. 3. The thermal and photochemical [2 þ 2] cycloaddition reaction of alkenes with carbonyls.The intramolecular nucleophilic substitution reaction -for example, the Williamson-type reaction -represents one of the important methods for preparing oxetane ring structures, and have been widely applied to the synthesis of oxetanes (Scheme 7.1) [10]. Unfortunately, side reactions -which include fragmentation from the intermediary alkoxide anion or elimination from the intermediary carbocation -often decrease the chemical yields of oxetane formation.The ring-expansion reaction of epoxides was first reported by Okuma and coworkers, to produce less-substituted oxetanes (Scheme 7.2) [11]. The nucleophilic attack by dimethyloxosulfonium methylide is proposed to react with the lessHandbook of Synthetic Photochemistry. Edited

Photochemical Approaches to Complex Chemotypes: Applications in Natural Product Synthesis

“… 154 − 159 A requirement for the Paternò–Büchi reaction to occur is that the alkene cannot have a lower triplet energy state than the carbonyl entity; if this would be the case, energy transfer is the major reaction pathway, with oxetane formation occurring slowly. 160 − 165 …”

Enantioselective synthesis of heterocyclic compounds using photochemical reactions

“…After photochemical excitation to the nπ* state, the 1,4-diradical intermediate 68 is formed. Such intermediates are also formed in the Paternò-Büchi reaction [132][133][134] where they play a key role for the stereoselectivity [21,135]. A typical reaction of such intermediates is the cleavage of the C-O bond (b) regenerating the substrate which diminishes the quantum yield of such reactions.…”

Oxetane Synthesis through the Paternò-Büchi Reaction

“…Only in 1953 Büchi reported the exact identification of the product, showing that this photochemical reaction could represent an interesting synthetic process [7]. After these pioneering works, several reviews have covered the relevant papers published in this field [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24].…”