Synthesis with 1,2-oxazines. 3. Reactions of .alpha.-chloroaldonitrones with enol ethers: a synthetic route to medium-ring lactones

Construction of Saturated Oxazolo[3,2-b][1,2]oxazines via Tandem [3+2]-Cycloaddition/[1,3]-Rearrangement of Cyclic Nitronates and Ketenes

“…NMR spectra in isomer series 5′ and 5″ revealed common features. The most characteristic was the position of the C-8a carbon in 13 C NMR, which was shifted downfield by 4−8 ppm for isomers 5′ compared to isomers 5″. This trend was used to assign the stereochemistry in cases when 2D NOESY was not informative (see Tables S1 and S2 there is a good correlation between calculated and experimental C-8a 13 C shifts in isomers 5′ and 5″, thus confirming the correctness of stereochemical assignments made.…”

“…This trend was used to assign the stereochemistry in cases when 2D NOESY was not informative (see Tables S1 and S2 there is a good correlation between calculated and experimental C-8a 13 C shifts in isomers 5′ and 5″, thus confirming the correctness of stereochemical assignments made. DFT calculations of 13 C NMR spectra were also used to assign the relative configuration of the major stereoisomer of product 5e, which is unsubstituted at C-8.…”

“…Slow decomposition of 3,3-dimethyl-substituted compounds 5 observed upon chromatography or storage at rt is likely to proceed through intermediates B, which undergo hydrogen transfer, followed by a retro-[4+2]-cycloaddition driven by the cleavage of the weak N−O bond. 13 The formation of decomposition products expected from this process (benzaldehyde and labile imine C) was detected for product 5e (see Scheme 7 and Supporting Information).…”

“…1 H NMR (300 MHz, CDCl 3 ) δ 7.12 (d, J = 8.6 Hz, 2H, o-C 6 H 4 OCH 3 ), 6.89 (d, J = 8.6 Hz, 2H, m-C 6 H 4 OCH 3 ), 3.89 (dd, J = 12.4, 8.0 Hz, 1H, 4-H), 3.82 (s, 3H, OCH 3 ), 2.15 (dd, J = 13.2, 8.0 Hz, 1H, 5-H), 2.01 (dd, J = 13.2, 12.4 Hz, 1H, 5-H), 1.93 (s, 3H, CH 3 CO), 1.46 (s, 3H, 7-C), 1.43 (s, 3H, 7-C). 13 [3,2-b][1,2]oxazin-2(3H)-one (5b′ and 5b″). Prepared from nitronate 2b (72 mg, 0.26 mmol) and dimethylketene according to the general procedure.…”

Silylation of bicyclic six-membered nitronates. Ring—chain tautomerism of intermediate N,N bis(oxy)iminium cations

“…This drawback can be eliminated by starting from bicyclic 5,6 dihydro 4H oxazines 1,6 or their N oxides, 7 in which the C(6) atom is bound to the rest of the molecule by an additional chain of atoms.…”