Condensation ether formation

Condensation reaction (Section 15 7) Reaction m which two molecules combine to give a product accompanied by the expulsion of some small stable molecule (such as water) An example is acid catalyzed ether formation... 

Condensation reactions between two hydroxymethyl substituents eliminate water to form ether linkages (Fig. 7.23a) or eliminate both water and formaldehyde to form methylene linkages (Fig. 7.23b). Ether formation is favored under neutral or acidic conditions and up to 130°C above which formaldehyde departs and methylene linkages are generated. The methylene linkage formation reaction, which eliminates water and fortualdehyde, is more prevalent under basic conditions. Condensation reactions between hydroxymethyl groups and reactive... 

These pre-condensates are most stable at pH 8-9 they are transformed by further condensation (essentially by elimination of water from hydroxymethyl groups and free NH groups) into poorly soluble and Anally insoluble, crosslinked products. Chemical modification of the soluble pre-condensate, for example, by esterification or ether formation, is again possible. 

Ketal regeneration proceeds step-wise through an enol ether intermediate. This intermediate is capable of several side reactions including a copper catalyzed aldol condensation, heavies formation and addition reactions with hydrochloric acid. Hydrochloric acid addition to l-methoxycyclohexene is of no consequence since this side reaction proved to be thermally reversible under oxycarbonylation reaction conditions. 

In the Sepracor synthesis of chiral cetirizine di hydrochloride (4), the linear side-chain as bromide 51 was assembled via rhodium octanoate-mediated ether formation from 2-bromoethanol and ethyl diazoacetate (Scheme 8). Condensation of 4-chlorobenzaldehyde with chiral auxiliary (/f)-f-butyl sulfinamide (52) in the presence of Lewis acid, tetraethoxytitanium led to (/f)-sulfinimine 53. Addition of phenyl magnesium bromide to 53 gave nse to a 91 9 mixture of two diastereomers where the major diasteromer 54 was isolated in greater than 65% yield. Mild hydrolysis conditions were applied to remove the chiral auxiliary by exposing 54 to 2 N HCl in methanol to provide (S)-amine 55. Bisalkylation of (S)-amine 55 with dichlonde 56 was followed by subsequent hydrolysis to remove the tosyl amine protecting group to afford (S)-43. Alkylation of (5)-piperizine 43 with bromide 51 produced (S)-cetirizine ethyl ester, which was then hydrolyzed to deliver (S)-cetirizine dihydrochloride, (5)-4. 

The condensation of the above substances in chloroform solution yields tellurium O-ethylpivalylacetone trichloride (II) as the main product, and tellurium 5/s-pivalylacetone dichloride (III) as a byproduct. The enolisation of the primary product (I), (CH3)3C.CO.CH2. CO.CH2.TeCl3, probably occurs with the CO-group remote from the tertiary group. The compound (I) cannot form a six-membered ring, but tends either to ether formation (II) or to condensation with a second molecule of pivalylacetone to give the dichloride (III). 

In the Sepracor synthesis of chiral cetirizine dihydrochloride (4), the linear side-chain as bromide 51 was assembled via rhodium octanoate-mediated ether formation from 2-bromoethanol and ethyl diazoacetate (Scheme 8). Condensation of 4-... 

S)-Proline was homologated to the pyrrolidinylacetic ester 864, after which N-acylation with p-methoxyphenylacetyl chloride, intramolecular condensation and enol ether formation created the indolizidinone system of 865. Standard transformations completed the synthesis of the alkaloid ( + )-(842), which was isolated as the hydrobromide salt ([a]o +54.1°, c 1, EtOH). 

How do reduction reactions that lead to a-CH2-groups affect pulping efficiencies Several typical (P-aryl ether) cleavage pathways would be blocked, which would have a negative effect on efficient delignification. However, because quinone methide formation would be prohibited, undesirable vinyl ether formation and condensation reactions would also be blocked. Bulk-phase fragmentation reactions, involving a... 

One of the difficulties in establishing the detailed chemistry of initial phase pulping is the dominance of the delignification rate by the first step in the process— formation of a quinone methide intermediate. The rates of subsequent steps are difficult to determine however, the rates of these steps are critical to the partitioning of the QMs toward fragmentation processes (p-aryl ether cleavage) as opposed to undesirable competing reactions, such as vinyl ether formation and condensation reactions [57]. 

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