A simple, versatile preparation of highly substituted 2,5-dihydro-oxoles

Abstract

Dialkyl-l-lithio-l -(phenylthio)allenes (1) add efficiently to ketones only on warming to -20 "C or above; the adducts are readily cyclised by acid, or several other electrophiles, to produce highly substituted 2,5-di hydro-oxoles.We have reported previously1 that the allenyl-lithium (1, R1 = H) is useful in the preparation of a-phenylthioenones. Initial attempts to extend this preparation, to other than methyl ketones, by use of 3,3-dialkylallenyl-lithiums~ (1, R1 = alkyl), were only partially successful since the yields of the ketone adducts (2, R1, R2 = alkyl) were poor (20-30%). Since large proportions of both the quenched allenyl-lithium and ketone were recovered unchanged, even after prolonged reaction times, it was evident that competitive enolisation was the problem.Removing hexamethylphosphoric triamide

An Alternative to Precious Metals: Hg(ClO4)2·3H2O as a Cheap and Water-Tolerant Catalyst for the Cycloisomerization of Allenols

“…Among the possibilities, transition-metal-catalyzed intramolecular nucleophilic addition of the hydroxyl group across the allene moiety in allenols is intriguing from the point of view of regioselectivity as well as it being one of the most rapid and convenient methods for the preparation of oxacycles owing to its atom-economy and efficiency . Despite the fact that that this field was initiated by mercury salts serving as promoters in stoichiometric or substoichiometric amounts, the discovery of platinum- and especially gold-based precatalysts , has displaced mercury, probably invoking toxicity issues . However, from a scientific point of view, prejudices coming from popularly accepted suppositions must be taken with care because a particular element can be toxic or not depending on the compound considered. , From an economic point of view, the common practice of using 5% loading in gold catalysis makes its use often impractical on larger scale synthesis in fields such as medicinal chemistry and material science.…”

Allenyl Sulfones and Allenyl Sulfides in the Synthesis of 3-Pyrrolines. A Novel Nucleophilic [3 + 2] Cycloaddition on Allenyl Sulfones Giving Rearranged Cycloadducts

“…Reaction of 4 (generated in situ from 1-phenyl thioprop-1-yne) with tosyl imine 2a gave the desired α-adduct 8 together with γ-adduct 9 (77% combined yield) (Scheme ). Subsequent 5- endo cyclization of 8 under AgNO 3 /K 2 CO 3 or AuCl catalysis afforded the corresponding 3-phenylthio-3-pyrroline 10 , which by oxidation with MMPP was converted into sulfone 5a .…”

A Convenient Synthesis of Tetrasubstituted Furans from Propargylic Dithioacetals

“…8b Hg(OAc) 2 has been shown to promote the annulation of allenyl carbinol 4 to form 2,5-dihydrofuran 5 with a HgOAc substituent at C-3 (Equation 1). 9 It is well documented that carbon-mercury bonds can be displaced by a number of substituents under different conditions. We envisioned that Hg(OAc) 2 might serve as an alternative route for the annulation of allenyl-carbinol intermediate 2 obtained from the reaction of propargylic dithioacetal with n-BuLi and then an aldehyde.…”

“…Equation 1 Cyclization of allenyl alcohol 4 by treatment with mercuric acetate. 9 Scheme 2 Approaches to the synthesis of tetrasubstituted furan derived from propargylic dithioacetal.…”