Condensation/intermolecular dehydration

Chemically amplified negative phenolic resists based on acid-catalyzed condensation/intermolecular dehydration cross-linking reactions... 

Chemically amplified condensation/intermolecular dehydration negative resists based on acid-catalyzed cross-linking with acid-sensitive electrophile (cross-linking agent)... 

The reaction is carried out in vapour phase (250°C) using a flow system (see methods section). This procedure turned out to be essential in order to mantain the hydrogen transfer as the main reaction pathway. A batch experiment carried out in an autoclave actually showed a wide range of condensation products besides some saturated ketone [6]. Reactions of ketones over oxide catalysts can lead to a variety of products due inter alia to aldol condensation, intramolecular dehydration and intermolecular disproportionation [16]. However, the presence of a good hydrogen donor such as a secondary alcohol and vapour phase conditions favour the transfer hydrogenation as the major reaction [16,17]. In our reaction conditions, products attributable to crotonic condensations and subsequent 1,4 Michael addition [18] were observed by g.l.c.-m.s. (Table 1). 

The general features of this elegant and efficient synthesis are illustrated, in retrosynthetic format, in Scheme 4. Asteltoxin s structure presents several options for retrosynthetic simplification. Disassembly of asteltoxin in the manner illustrated in Scheme 4 furnishes intermediates 2-4. In the synthetic direction, attack on the aldehyde carbonyl in 2 by anion 3 (or its synthetic equivalent) would be expected to afford a secondary alcohol. After acid-catalyzed skeletal reorganization, the aldehydic function that terminates the doubly unsaturated side chain could then serve as the electrophile for an intermolecular aldol condensation with a-pyrone 4. Subsequent dehydration of the aldol adduct would then afford asteltoxin (1). 

Fragmentation and recombination reactions predominate in nearneutral, alkaline solutions. Fragments from the cleavage of the carbohydrate, and from its dehydration products, undergo further dehydration, condensation, and intermolecular, Cannizarro-type reactions. The benzilic acid rearrangement, an intramolecular Cannizarro reaction, seems to be inoperative. 

BD conversion on Co-Zn/porcelain catalyst includes dehydrogenation, dehydration, intra- and intermolecular condensation, C-C bond destruction processes. The reaction products - 2,3-DHF, THF, 4-hydroxybutanal (4-HB) (1-2%),... 

Furthermore, the stability of two hydroxyl groups on a silicon atom is much greater than two hydroxyl groups on a carbon gem-diol, as these carbon compounds dehydrate to give ketones. Equation 11, or aldehydes. Equation 12, while the corresponding silanediols give rise to the formation of po 1 ysi 1 oxanes via intermolecular condensation. 

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