Cj-synthon

Usually, the a-pyranic addition products (3) cannot be isolated because they isomerize immediately in a thermally allowed electrocyclic process affording acyclic dienones (4), which in many cases react further. Therefore, pyrylium salts 2A react with most nucleophiles as if they had the electronic structure 2B, and thus the pyrylium cation behaves as a useful synthon, namely the last vinylog in the series of acyl halides (Cj synthon) - p-halovinyl ketones (C3 synthon) - pyrylium (C5 synthon) [50, 51],... 

Some selected alkaloids are readily accessible via the corresponding chromium carbenes. A versatile synthesis of indolocarbazoles 133 and 134, bioactive natural products with interesting protein kinase C inhibiting properties, is based on photobenzannulation. Depending on the nature of the Cj-synthon, dioxy and oxyamino derivatives have been synthesized in good to very good yields (Scheme 46) [96]. 

Perhaps the best known example of the constructive application of a destructive process is the set of classical methods employing acetoacetic and malonic esters. In these methods, a decarboxylation step, the rupture of a C-C bond, is usually required after the initial formation of the new C-C bond by the alkylation. The ease of this step is actually a prerequisite for the nearly universal application of acetoacetic and malonic esters as synthetic equivalents of or Cj synthons. 

The heterocyclic systems of 1,3-oxazinium ions are widely used in the organic synthesis . Thus, 5,6-dihydro-4/f-l,3-oxazines 40, being a cyclic derivative of 1,2-aminoalcohols, can serve as Cj-synthones for the preparation of aldehydes, ketones and carboxylic acids . The 47/-l,3-oxazines 47 can undergo oxidative dehydrogenation by treatment with the trityl salts or benzoquinone to form the 3-azapyrylium salts 48, which then give various jV-acylenamine derivatives 49-52 by reaction with water or with active methylene comjKJunds (equation 18) . 

These reactions are summarized in Table V and collected according to the Cj-synthons used and the substituents R entering the newly formed triazole ring. Superscripts to references indicate formation of 1,2,4-triazoIo[4,3-fl] (a) and -[1,5-ulpyrimidines (b) a description of the re-... 

Hu et al. (91 Mil) report the Dimroth rearrangement of 2-hydrazino- to 2,3-diamino quinazolin-4-ones under catalysis of carboxylic acid derivatives this reaction is followed by an in situ cyclization in which the same carboxylic acid derivatives serve as Cj-synthons. l-Acylamino-2-alkylthiopyrimidines are hydrazinolyzed and cyclized to 3-amino-1,2,4-triazolo[l,5-a]pyrimidinium salts (89EGP270711). 

Trimethylsilyldiazomethane, as a stable and safe substitute for hazardous diazomethane, is useful both as a reagent for introducing a Cj-unit and as a C-N-N synthon for the preparation of azoles. Many methods are described in the literature for the preparation of trimethylsilyldiazomethane, including the trimethylsilylation of diazomethane (7-74S), the alkaline decomposition of N-nitroso-N-(trimethylsilylmethyl)amides (25-61%) and the diazo group transfer reaction of trimethylsilylmethyllithium with p-toluenesulfonyl azide (38%). The present modified diazo group transfer method appears to be the most practical, high-yield, and large scale procedure for the preparation of... 

QM Gu, CS Chen, CJ Sih. Bifunctional chiral synthons via biochemical methods VIII. Optically active 3-aroyl-thio-2-methylpropionic acid. Tetrahedron Lett 27 1763-1766, 1986. 

The benzannulation product may be viewed as a formal [3+2+1] cycloaddition product, in which the alkyne (C2 synthon) is connected to the carbon monoxide ligand (Cj unit) and the carbene carbon atom of the unsaturated carbene ligand (C3 building block), the / -carbon... 

For the synthesis of (-)-N-methylmaysenine, the synthon resulting from disconnections B and C was in practice a chiral Cg compound obtained from tri-0-acetyl-D-glucal which was transformed to an open chain dithian reactant X as shown in route (c). Reaction of X with the aryktienal P (step xii) led to Q which was then chain extended successively by C3 and then Cj to afford the two rings in the maysenine ring structure as shown in route (d). 

In the total synthesis of racemic N-methylmaysenine the synthon component from disconnections B and C had been derived (ref. 281) from cis-2-butene-1,4-diol by conversion to the dimethylacetonide by reaction with 2,2-dimethoxypropane followed by five further reactions although all the remaining reactions with the arykJienal and the C3 and Cj chain extentions were basically the same as in the synthesis of the (-) enantiomer. 

Oxazolines are nowadays essential ligands in asymmetric catalysis and also important synthons for stereoselective synthesis [8]. The success of the Cj-symmetric bis(oxazolines) ( BOX ) and pyridine-bis(oxazolines) ( Pybox ) discovered in the early 1990s has established them as a privileged class of ligands [9]. In contrast, the development and application of trisoxazolines lagged behind for a long time. Katsuki and collaborators [10] reported the first example of a chiral trisoxazoline in 1995 and their use in the allylic oxidation of alkenes (Kharasch-Sosnovsky reaction), as well as the enantioselective addition of diethylzinc to aldehydes. 

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