Boronic Ester Enabled [2 + 2]-Cycloadditions by Temporary Coordination: Synthesis of Artochamin J and Piperarborenine B

Abstract

A strategy for the photosensitized cycloaddition of alkenylboronates and allylic alcohols by a temporary coordination is presented. The process allows for the synthesis of a diverse range of cyclobutylboronates. Key to development of these reactions is the temporary coordination of the allylic alcohol to the Bpin unit. This not only allows for the reaction to proceed in an intramolecular manner but also allows for high levels of stereo and regiocontrol. A key aspect of these studies is the utility of the cycloadducts in the synthesis of complex natural products artochamin J and piperarborenine B.

Photochemical [2+2] Cycloaddition of Alkynyl Boronates

“…Cyclobutane‐derived boronates and their preparation are broadly described in the literature [23–42] . On the contrary, the synthesis of cyclobutene‐derived boronates and their applications are very scarce.…”

“…Thus, lithiation of iodo‐ ( 1 a ) [43] or bromocyclobutene ( 1 b ) [44,45] and subsequent treatment with trialkoxy borates resulted in ionic boronate 2 a , [43] or in neopentyl‐protected boronate 2 b after transesterification, [44,45] respectively (Scheme 1, A ). The same strategy was used to prepare the simplest cyclobutene derivative 2 c , albeit with moderate overall yield [42] . Miyaura‐type alkenyl triflate borylation was used for the preparation of boronates 3 a and 3 b (Scheme 1, B ) [19] .…”

“…[18,21,22] Cyclobutane-derived boronates and their preparation are broadly described in the literature. [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] On the contrary, the synthesis of cyclobutene-derived boronates and their applications are very scarce. Thus, lithiation of iodo-(1 a) [43] or bromocyclobutene (1 b) [44,45] and subsequent treatment with trialkoxy borates resulted in ionic boronate 2 a, [43] or in neopentyl-protected boronate 2 b after transesterification, [44,45] respectively (Scheme 1, A).…”

“…The same strategy was used to prepare the simplest cyclobutene derivative 2 c, albeit with moderate overall yield. [42] Miyaura-type alkenyl triflate borylation was used for the preparation of boronates 3 a and 3 b (Scheme 1, B). [19] In addition, formation of the cyclobutenyl boronate core was observed as a side product in the reaction of boronate 4 a with Fischer carbene complex 5 (Scheme 1, C).…”

Access to Azetidines via Gold Mediated Energy Transfer Photocatalysis

“…Four membered rings, and among them azetidines, hold great value, but have for long been tedious to access . However, recent progress in [2 + 2] photocycloadditions has created new methods toward four-membered ring construction . Facing a very rapid development as a common synthetic tool, photocatalysis has for long focused mainly on photoredox pathways through Single Electron Transfer (SET), but in the past few years, Energy Transfer (EnT) has also emerged as a promising mechanism for photocatalysis and for molecular assembly construction …”