Direct Synthesis of Benzo[b]fluorenyl Thiophosphates via Tandem Cyclization of Diynols with (RO)₂P(O)SH

Abstract

A general and metal-free protocol for the construction of benzo­[b]­fluorenyl thiophosphates was developed through the cascade cyclization of easily prepared diynols and (RO)2P­(O)­SH, with water as the only byproduct. The novel transformation involved the allenyl thiophosphate as the key intermediate, followed by Schmittel-type cyclization to achieve the desired products. Notably, (RO)2P­(O)­SH acted not only as a nucleophile but also as an acid-promoter to initiate the reaction.

Cascade Cyclization of 1,5-Diynols and (RO)2P(O)SH to Construct Benzo[b]fluorenyl S-Alkyl Phosphorothioates under Catalyst-Free Conditions

“…Based on the above experimental results and previous literature reports, , a possible reaction mechanism for this cascade cyclization was proposed (Scheme ). Initially, the dehydration of 1,5-diynol 1 , with (EtO) 2 P­(O)­SH as the acid promoter, gave propargylic carbocation intermediate A , which readily resonated to allenyl carbocation intermediate B .…”

“…Additionally, the hydrolysis of the C-SP­(O)­(OEt) 2 bond of the obtained product 3a followed by oxidation could smoothly afford the benzo­[ b ]­fluorenone 8 in moderate yield under alkaline conditions, and the benzo­[ b ]­fluorenol 8′ can also be obtained with low yield (Scheme -2). Instead of generating the expected disulfide product …”

“…Recently, we reported an efficient approach for the direct synthesis of benzo­[ b ]­fluorenyl S -aryl phosphorothioates from 1,6-diynols with (RO) 2 P­(O)­SH under catalyst- and additive-free conditions (Scheme ). In this novel transformation, allenyl thiophosphate was the key intermediate, which was then involved in a Schmittel-type cyclization to afford the target product. Inspired by this novel transformation and the continuing interest in the cascade reactions of propargylic alcohols, we speculated that the rapid construction of benzo­[ b ]­fluorenyl S -alkyl phosphorothioates could be achieved by performing the cascade cyclization of 1,5-diynols with (RO) 2 P­(O)­SH as the acid and nucleophile.…”