BF₃‐Catalyzed Synthesis of Cyclic Carbamates from Boc‐Protected Aminals and Alkynes

Abstract

Cyclic carbamates were synthesized by the acid‐catalyzed reaction between Boc‐protected aminals as an imine precursor and 4‐methoxyphenyl‐substituted alkynes. The resulting cyclic carbamates could be converted to the one carbon ring‐contracted cyclic carbamates by ozonolysis. Consequently, 4‐methoxyphenyl‐substituted alkynes served as masked acyl anion equivalents.

A Ag2CO3/TFA-catalyzed intramolecular annulation approach to imidazo[1,2-c][1,3]oxazin-5-one derivatives

“…In order to prepare a library of new imidazo[1,2- c ][1,3]oxazin-5-one heterocycles, N -Boc-4-bromo-5-methyl-2-phenylethynylimidazole 1a was studied as a model substrate and subjected to some annulation process conditions which have been already described in the literature. 25,37,46,49,50,57–64 Compound 1a , which is a suitable substrate for undergoing intramolecular annulation, was prepared following our recent procedure involving the regioselective Sonogashira cross coupling reaction of N -Boc-2,5-dibromo-4-methylimidazole. 48 Initially, the zinc catalyst (ZnCl 2 ) was used in dichloromethane at 40 °C but it was found to be totally inert since the starting material was completely recovered after 48 h (Table 1, entry 1).…”

AgNTf2 catalyzed cycloaddition of N-acyliminium ions with alkynes for the synthesis of the 3,4-dihydro-1,3-oxazin-2-one skeleton

“…The Lewis acid-catalyzed intramolecular addition–cyclization of N -acyliminium ions with alkynes is widely utilized in organic synthesis, 12 and the intermolecular version of such addition–cyclization has gradually attracted more attention in recent years. 13 For example, SnCl 4 , 14 Bi(OTf) 3 12 c and BF 3 ·OEt 2 15 could mediate the intermolecular carboarylation of N , O -acetals or N , N -acetals with alkynylarenes (Fig. 2).…”

“…21 It is worth noting that TiCl 4 could mediate the addition of cyclic N , O -acetals with terminal alkynes to give addition–chlorination products. 22 To the best of our knowledge, the intermolecular cycloaddition of chain N , O -acetals with unsaturated carbon–carbon bonds is quite rare, with one example for alkynes (SnCl 4 mediated 14 ) and another example for olefins (anodic oxidation 15 ). Herein, we present our AgNTf 2 -catalyzed [4 + 2] cycloaddition of chain N , O -acetals with terminal alkynes, providing an efficient synthetic approach to 4,6-substituted 3,4-dihydro-1,3-oxazin-2-ones.…”

Synthesis of a 3,4-Dihydro-1,3-oxazin-2-ones Skeleton via an Intermolecular [4 + 2] Process of N-Acyliminium Ions with Ynamides/Terminal Alkynes

“…Another example is 1,3-oxazinan-2-one 3 , which is also an interesting scaffold within natural products and pharmaceuticals including anti-HIV,, antidiabetic,, anti-inflammatory and anticancer,, bioactive molecules. Although numerous synthetic approaches have been achieved to access skeleton 3 in past years, there are very few methods to obtain substituted 3,4-dihydro-1,3-oxazin-2-one 4 , which employed acid-catalyzed cyclization of Boc-protected aminals with alkynes …”