Herein we report the polymer-supported synthesis of 3,4-dihydro-2H-1,4-oxazine-3-carboxylic acid derivatives using immobilized Fmoc-Ser(tBu)-OH and Fmoc-Thr(tBu)-OH as the starting materials. After the solid-phase-synthesis of N-alkyl-N-sulfonyl/acyl intermediates, the target dihydrooxazines were obtained using trifluoroacetic acid-mediated cleavage from the resin. This approach was also studied for the preparation of dihydrothiazines from immobilized Fmoc-Cys(Trt)-OH. Inclusion of triethylsilane in the cleavage cocktail resulted in the stereoselective formation of the corresponding morpholine/thiomorpholine-3-carboxylic acids. Stereochemical studies revealed the specific configuration of the newly formed stereocenter and also the formation of stable N-acylmorpholine rotamers.
Herein, we report a simple synthesis of disubstituted pyrazino‐oxazines with controlled stereochemistry using readily available building blocks: N‐Fmoc‐protected α‐amino acids and 2‐bromo ketones. The convenient solid‐phase synthesis afforded polymer‐supported derivatives of N‐alkylated N‐acylated tert‐butylserine, which were subjected to spontaneous cyclative cleavage to yield the corresponding pyrazine intermediates. The target 1,7,8,9a‐tetrahydropyrazino[2,1‐c][1,4]oxazine‐6,9‐diones were obtained in two steps by acid‐mediated cleavage of the tert‐butyl group followed by cyclization of the oxazine scaffold.
In this review, we summarize synthetic approaches to preparing single or fused oxazine and thiazine derivatives using solid-phase synthesis (SPS). The literature survey revealed that diverse compounds bearing variously functionalized 1,2-oxazine, 1,3-oxazine, or 1,4-oxazine scaffolds and the corresponding thiazines are accessible by SPS. The latest contributions involving the stereoselective polymer-supported syntheses of morpholines indicate that the field is continuing to expand.
N-(3-Phenylprop-2-yn-1-yl)-sulfonamides
derived
from serine and threonine were synthesized using solid-phase synthesis
and subjected to reaction with trimethylsilyl trifluoromethanesulfonate
(TMSOTf). In contrast to the previously reported formation of 1,4-oxazepanes,
this reaction afforded pyrrolidin-3-ones. A mechanistic explanation
for this unexpected outcome is proposed, and the limitations and scope
of the rearrangement are outlined.
Herein, we report a stereoselective formation of tetrahydro-6H-benzo[e][1,4]oxazino[4,3-a][1,4]diazepine-6,12(11H)-diones. Their preparation consisted in solid-phase synthesis of linear intermediates starting from polymer-supported Ser(tBu)-OH. Using various 2-nitrobenzoic acids and bromoketones, the key intermediates were obtained in five steps and subjected to trifluoroacetic acid-mediated cleavage from the resin, followed by stereoselective reduction with triethylsilane. Subsequent catalytic hydrogenation of the nitro group and cyclization yielded the target compounds with full retention of the C stereocenter configuration.
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