Deciphering substitution effects on reductive hydroalkoxylation of alkynyl aminols for stereoselective synthesis of morpholines and 1,4-oxazepanes: total synthesis of tridemorph and fenpropimorph

Acid catalysed reductive etherification of N-propargyl amino alcohols for the stereoselective synthesis of cis-2,5/2,6-disubstituted morpholines and cis-2,6/2,7-disubstituted oxazepanes has been developed. Mechanistic studies revealed that terminal alkynols gave morpholines via a 6-exo-dig hydroalkoxylation-isomerization-reduction cascade. Interestingly, an alkyne hydration-cyclization-reduction sequence is found to be involved in the formation of oxazepanes from alkyl substituted internal alkynols. The strategy was used as a key step in the total synthesis of fungicides tridemorph and fenpropimorph.

Conceptual design and cost-efficient environmentally Benign synthesis of beta-lactams

Stereoselective preparation of diverse trans and cis β-lactams following different experimental conditions are executed. A variety of circumstances are critically analyzed. It has been found that the stereochemistry of the products depends on a number of parameters including the conditions of the procedures, composition of the Schiff bases and acid chlorides or equivalents, method of addition of the reactants, temperature of the process and nature of the media. Using some of the compounds and methods as described herein, a number of useful chemical transformations for the preparation of heterocycles are achieved. These methods include indium-catalyzed glycosylation of amino β-lactams, preparation of pyrrole-substituted β-lactams, cycloaddition with sterically congested Schiff bases towards β-lactams, Michael reaction for the preparation of polycyclic oxazepenes and synthesis of two chiral isomers of the thienamycin side chain. Most of the products are obtained stereospecifically and in optically active forms. Many reactions described here are catalytic and therefore, these are environmentally friendly.

Synthesis of chiral 1,4-oxazepane-5-carboxylic acids from polymer-supported homoserine

The preparation of novel 1,4-oxazepane-5-carboxylic acids bearing two stereocenters is reported in this article. Fmoc-HSe(TBDMS)-OH immobilized on Wang resin was reacted with different nitrobenzenesulfonyl chlorides and alkylated with 2-bromoacetophenones to yield N-phenacyl nitrobenzenesulfonamides. Their cleavage from the polymer support using trifluoroacetic acid (TFA) led to the removal of the silyl protective group followed by spontaneous lactonization. In contrast, TFA/triethylsilane (Et₃SiH)-mediated cleavage yielded 1,4-oxazepane derivatives as a mixture of inseparable diastereomers. The regioselectivity/stereoselectivity depended on the substitution of the starting 2-bromoacetophenones and was studied in detail. Catalytic hydrogenation of the nitro group improved the separability of the resulting diastereomeric anilines, which allowed us to isolate and fully characterize the major isomers.

The preparation of novel 1,4-oxazepane-5-carboxylic acids bearing two stereocenters is reported in this article.

Expeditious diastereoselective synthesis of medium ring heterocycle-fused chromenes via tandem 8/9-endo-dig and 8-exo-dig hydroalkoxylation-formal-[4 + 2] cycloaddition

The first examples of highly diastereoselective tandem 8/9-endo-dig and 8-exo-dig hydroalkoxylation-formal-[4 + 2] cycloaddition are described for the synthesis of medium ring heterocycle-fused chromenes. TMS-alkynols preferred the exo-dig mode of hydroalkoxylation over the endo-dig mode leading to spiro-cyclic chromenes. The method could be used for the synthesis of linearly-fused ladder-like polyethers. A thia-heterocycle-fused chromene could be transformed into a complex bridged tricyclic ketal by a tandem carbene-insertion-[2,3]-sigmatropic shift.

An efficient synthesis of 16H-dibenzo[2,3:6,7][1,4]oxazepino[5,4-b]quinazolin-16-ones via an Ullmann reaction catalyzed by CuI

CuI functions as a mild Lewis acid catalyst to promote the condensation and cyclization reaction of 2-amino-N-(2-hydroxyphenyl)benzamide and 2-bromobenzaldehyde to build the quinazoline moiety first. With the addition of Cs₂CO₃, it also can catalyse the subsequent intramolecular Ullmann type reaction to give 16H-dibenzo[2,3:6,7][1,4]oxazepino[5,4-b]quinazolin-16-ones in good yields.

Synthesis of Benzo[1,4]heterocycles using Palladium Catalyzed Heck Reaction to Vinylogous Carbonates/Carbamates: Unexpected Formation of Indoles via Carbopalladation Intercepted by Nucleopalladation

An efficient protocol for the stereoselective synthesis of benzo[1,4]heterocycles via palladium catalyzed Heck reaction on o-halo-aryl-oxa/thia/aza tethered vinylogous carbonates/carbamates/esters has been developed. Unexpected formation of indoles is observed when unprotected 2-iodoaniline tethered vinylogous carbonates are subjected to the Heck reaction. Mechanistic studies indicate that formation of these indoles is an outcome of the interception of the carbopalladation step by nucleopalladation. The method can be used to gain rapid access to the core skeleton of abacopterin A-C.

Alkyl Aryl Ether Bond Formation with PhenoFluor

An alkyl aryl ether bond formation reaction between phenols and primary and secondary alcohols with PhenoFluor has been developed. The reaction features a broad substrate scope and tolerates many functional groups, and substrates that are challenging for more conventional ether bond forming processes may be coupled. A preliminary mechanistic study indicates reactivity distinct from conventional ether bond formation.

Oxidative C–S bond cleavage reaction of DMSO for C–N and C–C bond formation: new Mannich-type reaction for β-amino ketones

A novel oxidative C–S bond cleavage reaction of DMSO for N-methylation and subsequent C–C bond formation is described.

Copper catalysed [3 + 2] cycloaddition with concomitant annulation: formation of 2,4-diaryl-1,4-oxazepan-7-ones via a ketenimine route

A novel strategy of copper catalyzed cascade reaction involving intramolecular nucleophilic addition to N-sulfonylketenimine gratifyingly furnishing 1,4-diaryl oxazepan-7-one has been described.