Beta Cleavage and Condensation

The following are topics that may be especially valuable to the student and which might be read initially in Chapter 12, lysozyme (Section B,5), chymo-trypsin (Section C,l), kinases (Section D,9), multiple displacement, reactions (Section G) in Chapter 13, imines (Section A,2), addition to C=C bonds (Section A, 4,5), beta cleavage and condensation (Section C) in Chapter 14, thiamin diphosphate (Section D), pyridoxal phosphate (Section E) in Chapter 15, NAD (Section A). 

Vitamins are classified by their solubiUty characteristics iato fat-soluble and water-soluble groups. The fat-soluble vitamins A, E, and K result from the isoprenoid biosynthetic pathway. Vitamin A is derived by enzymic cleavage of the symmetrical C q beta-carotene, also known as pro-vitamin A. Vitamins E and K result from condensations of phytyldiphosphate (C2q) with aromatic components derived from shikimic acid. Vitamin D results from photochemical ring opening of 7-dehydrocholesterol, itself derived from squalene (C q). 

The reaction involves nucleophilic attack at the beta cyclopropyl carbon and cleavage of the cyclopropyl ring which produces an ylid that condenses intramolecularly with the carbonyl moiety. 

In catalytic cracking many reactions take place simultaneously. Cracking occurs by C-C bond cleavage of paraffins, dealkylation etc. Isomerization and even condensation reactions take place. These reactions occur via positively charged hydrocarbon ions (carbocations). The nature of the carbocations is the subject of debate. For the cracking of paraffinic hydrocarbons it is usually assumed that carbenium ions are the crucial intermediates, which decompose via beta fission into olefins and (smaller) carbenium ions (see Chapter 4, Section 4.4). A typical reaction mechanism for catalytic cracking (and hydrocracking) imder the relatively mild conditions used in FCC is shown overleaf. 

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