Dehydration intermolecular

Dehydration to olefins, which sometimes accompanies the reaction of alcohols with DAST [95, 108], is seldom as extensive as with a-fluoroamines (FAR and 1,1,2,3,3,3 hexafluoropropyldiethylamine) but occurs in a few cases to the exclusion of fluonnation, thus, 9a-fluoro-11-hydroxysteroids give 9a fluoro-A -steroids [127, 128] Dehydration accompanied by Wagner-Meerwein rearrangement occurs during the fluonnation of testosterone [129] Intermolecular dehydration to form ethers in addition to fluorides is observed in the reaction of benzhydryl alcohols [104] (Table 6)... 

The strained bicyclic carbapenem framework of thienamycin is the host of three contiguous stereocenters and several heteroatoms (Scheme 1). Removal of the cysteamine side chain affixed to C-2 furnishes /J-keto ester 2 as a possible precursor. The intermolecular attack upon the keto function in 2 by a suitable thiol nucleophile could result in the formation of the natural product after dehydration of the initial tetrahedral adduct. In a most interesting and productive retrosynthetic maneuver, intermediate 2 could be traced in one step to a-diazo keto ester 4. It is important to recognize that diazo compounds, such as 4, are viable precursors to electron-deficient carbenes. In the synthetic direction, transition metal catalyzed decomposition of diazo keto ester 4 could conceivably furnish electron-deficient carbene 3 the intermediacy of 3 is expected to be brief, for it should readily insert into the proximal N-H bond to... 

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). 

Levitt (1962) has of course proposed a theory of frost injury based on the oxidation of sulphydryl groups and the formation of intermolecular disulphide bonds and he has subsequently extended this concept to dehydration injury (see Levitt, 1972). Although popular in the 1960s and 1970s this theory appears to have lost favour in recent years. 

Attempts to synthesize ethers with 2° alkyl groups by intermolecular dehydration... 

Intermolecular cross aldolization of metallo-aldehyde enolates typically suffers from polyaldolization, product dehydration and competitive Tishchenko-type processes [32]. While such cross-aldolizations have been achieved through amine catalysis and the use of aldehyde-derived enol silanes [33], the use of aldehyde enolates in this capacity is otherwise undeveloped. Under hydrogenation conditions, acrolein and crotonaldehyde serve as metallo-aldehyde enolate precursors, participating in selective cross-aldolization with a-ketoaldehydes [24c]. The resulting/ -hydroxy-y-ketoaldehydes are highly unstable, but may be trapped in situ through the addition of methanolic hydrazine to afford 3,5-disubstituted pyridazines (Table 22.4). 

The intermolecular Heck reaction of 3-bromofuran and tosylallyamine 88 gave adduct 89 under the classical Heck conditions [79], Subsequent Rh-catalyzed hydroformylation with ring closure occurred regioselectively to furnish the hydroxypyrrolidine, which was dehydrated using catalytic HC1 to afford dihydropyrrole 90. 

Isosorbide Dinitrate Isosorbiddinitrate, 1,4 3,6-dianhydrosorbate-2,5-dinitrate (19.1.4), is synthesized by intermolecular dehydration of D-sorbite into isosorbide (19.1.3) using para-toluenesulfonic acid and subsequent nitration of the two hydroxyl groups by nitric acid [5-7]. 

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. 

Problein 14.9 Although benzyl alcohol, C HjCHjOH, is a T alcohol, it undergoes intermolecular dehydration by both S 2 and S l mechanisms. Explain. M... 

Problem 14.12 Use any needed starting material to synthesize the following ethers, selecting from among intermolecular dehydration, Williamson synthesis, and alkoxymercuration-demercuration. Justify your choice of method. 

Phenols do not undergo intermolecular dehydration. Although aryl halides cannot be used as substrates in typical Williamson syntheses, they do undergo a modified Williamson-type synthesis at higher temperature in the presence of Cu. 

Concerning the membrane itself, phospholipid hydration was characterized in terms of the types of water, bound and bulk, and their exchange rates, as well as direct observation of the intermolecular contacts between the phosphate headgroup and bound water via HRMAS HOESY and between lipids in mixed membranes via HRMAS NOESY.112 Significantly, Zhou and co-workers found little dehydration of the membranes even when rotation rates as high as 9 kHz were used, providing some comfort that the centripetal forces of sample rotation are not changing the structure of the membrane. 

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