Of p-methoxybenzyl ethers, to form

It is known [37] that the DDQ-oxidation of a p-methoxybenzyl ether 45 generates an oxonium ion 46 in its low energy conformation in which the two hydrogen atoms are in a syn disposition. This conformation of the oxonium ion avoids 1,3-ally lie strain [26]. Cyclization via this conformation under non equilibrated conditions leads directly to the desired acetal 41. Once 41 was formed, we had to avoid acidic conditions during the rest of the synthesis. We were therefore dismayed when the dichloro-dicyano-hydroquinone formed as a coproduct on DDQ-oxidation of the acetal 32 led to an epimerization of the C9/C11-dioxane The resulting product 48 cannot be recycled and is completely worthless for the synthesis of erythronolide. 

The behavior of the 3 -pyridyl-, 3 -indolyl- and 2 -pyrrolyl-substituted j8-nitro-ethenes (31a-c) in the domino cycloaddition was also studied [23]. At 15 kbar, 18 h and RT, the / -nitroethenes (31a-c) reacted efficiently with p-methoxybenzyl vinyl ether (14) and methyl acrylate to form the bicyclic nitronates (32a-c) (Scheme... 

Typical procedure. p-Methoxybenzyl carbamates of amino acids 546 [388] The reagent 545 was prepared as follows a solution of anisyl alcohol (12.4 mL, 0.1 mol) in dry diethyl ether (100 mL) was added to a solution of phosgene (for a safe source, see Chapter 7) (21 mL, 0.3 mol) in dry diethyl ether (100 mL) over a period of 10 min. Then, a solution of dimethylaniline (12.7 mL, 0.1 mol) in dry diethyl ether (100 mL) was added dropwise over a period of 1 h and the resulting mixture was stirred for 2 h at —10 to —5 °C. After removal of the salt formed in the reaction by filtration, the filtrate was concentrated under reduced pressure below 0 °C to leave an oily residue. Twice more, dry diethyl ether (100 mL) was added and evaporation was repeated to remove excess phosgene. The residue was immediately dissolved in tetrahydrofuran (70 mL) to give a solution ready for use in the next reaction. 

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