Synthetic versatility

The acyl derivatives of A-hydroxypyridine-2-thione are a synthetically versatile source of free radicals. These eompounds are readily prepared from reactive acylating agents, sueh as acyl chlorides, and a salt of A-hydroxypyridine-2-thione ... 

Core Functional Group. A member of the most simple, fundamental, and synthetically versatile class of functional groups, such as carbonyl, hydroxyl, olefinic, acetylenic, and amino groups. 

Synthetic versatility of 2-oxazolone heterocycle for stereocontrolled construction of 2-amino alcohols 97YGK1018. 

The intramolecular cycloaddition of a nitrile oxide (a 1,3-dipole) to an alkene is ideally suited for the regio- and stereocontrolled synthesis of fused polycyclic isoxazolines.16 The simultaneous creation of two new rings and the synthetic versatility of the isoxa-zoline substructure contribute significantly to the popularity of this cycloaddition process in organic synthesis. In spite of its high degree of functionalization, aldoxime 32 was regarded as a viable substrate for an intramolecular 1,3-dipolar cycloaddition reaction. Indeed, treatment of 32 (see Scheme 17) with sodium hypochlorite... 

In this paper the scientific and technologic relevance of poly(organophos-phazenes) are accounted for on the basis of the synthetic versatility of these materials and of their structure-property relationships. 

Siegel, H. Lithium Halocarbenoids Carbanions of High Synthetic Versatility. 106, 55-78 (1982). 

A more recent approach, which also profits from the synthetic versatility of stabilized thionium ions, has been elaborated by Berard and Piras [22]. These authors observed that the cyclobutane thionium ions 1-76 obtained from the cyclopropyl phenyl sulfides 1-75 by treatment with pTsOH under anhydrous conditions can be trapped by an adjacent electron-rich aromatic ring to give the chromane derivatives 1-77 in good to excellent yields (Scheme 1.20). As expected, 1-77 were obtained as single diastereoisomers with a ds-orientation of the methyl and the phenylthio group as a consequence of steric constraints. 

As shown in Fig. 25b, the systematic tuning of emission wavelength was achieved by the combinatorial introduction of substitutents at the two diversity points on the fluorescent core skeleton. In addition to the synthetic versatility and predictability on emission wavelengths, these novel fluorophores were compatible with the modification of biopolymers and successfully applied in the immunofluorescence (see Fig. 25c). 

The introduction of heterodienes has extended the synthetic versatility of cycloaddition reactions in organic synthesis.150 Denmark and coworkers have developed the use of nitroalkenes as dienes in [4+2] cycloaddition. Nitroalkenes react with simple alkenes in the presence of SnCl4 as a promoter. For example, the reaction of nitrocyclohexene with cyclopentene gives three products. The major product is awh-isomer, which arises from an exo approach of cyclopentene toward nitrocyclohexene (see Eq. 8.94).151... 

The synthetically versatile 6-amino-7-cyano-l,4-dihydro-3-methyl-l-phenyl-4-thioxothiopyrano[4,3-c]pyrazole 107 has been used for the synthesis of several bicyclic and tricyclic systems. For example, reaction with hydrazine gives 108, which can be used to generate tricycles 109-111 (Scheme 8) <2002H(57)1121>. 

Direct attachment of a functional group to the carbon atoms of the cyclopropane ring is fairly difficult. gem-Dibromocyclopropanes are readily available through the addition of dibromocarbene to olefinic compounds. Their synthetic versatility is reviewed in a previous volume of this series [77]. Substitution of the bromide with the aid of nucleophilic organometallics to form a new carbon-carbon bond has been investigated [78]. 

Benzocyclobutene, when generated by oxidation of its iron tricarbonyl complex, can function as the dipolarophile in 1,3-dipolar cycloaddition reactions with arylnitrile oxides (Scheme 113).177 Unfortunately the synthetic versatility of this type of process is limited because of the unreactivity of other 1,3-dipolar species such as phenyl azide, benzonitrile N-phenylimide, and a-(p-tolyl)benzylidenamine N-oxide.177... 

This section diverges from the main subject but, being studied together with the insertion of (phenylthio)carbenes with alkoxides, is worth describing here due to its synthetic versatility as C-C bond forming reactions. 

In particular, Diels-Alder adducts from the enantioselective reaction of 2-bromo-acrolein and 2-chloroacrolein with a variety of dienes are of exceptional synthetic versatility. Readers are advised to consult the review article by Corey and Guzman-Perez.54 For the purpose of quick reference, chiral ligands commonly used in Diels-Alder reactions are listed in Table 5-6. 

The synthetic versatility and significance of the Zr-catalyzed kinetic resolution of exocyc-lic allylic ethers is demonstrated by the example provided in Scheme 6.9. The optically pure starting allylic ether, obtained by the aforementioned catalytic kinetic resolution, undergoes a facile Ru-catalyzed rearrangement to afford the desired chromene in >99% ee [20], Unlike the unsaturated pyrans discussed above, chiral 2-substituted chromenes are not readily resolved by the Zr-catalyzed protocol. Optically pure styrenyl ethers, such as that shown in Scheme 6.9, are obtained by means of the Zr-catalyzed kinetic resolution, allowing for the efficient and enantioselective preparation of these important chromene heterocycles by a sequential catalytic protocol. 

When, furthermore, phenols (368) are coupled with 1 in the presence of a Pd° catalyst, the phenoxy-methyl-1,3-dienes 369 are produced [158]. As aryl allyl ethers, these can be made to undergo a Claisen rearrangement (205 °C, DMF) and the ensuing 2-(l,3-dienylmethyl)phenols 370 finally cydize in the presence of a trace of acid to a mixture of exo-methylene chromans 371 (major product) and dihydrobenzofur-ans 372 - a remarkable generation of functional and structural complexity from simple starting materials with 100% atom economy and underlining impressively the synthetic versatility of modern allene chemistry ... 

Due to the synthetic versatility of donor-substituted allenes, it is of great importance to modify these compounds by substitution reactions at C-l. The CH-acidity at the a-carbon of alkoxyallenes allows their smooth lithiation [12, 42] which conveniently... 

Hetero Pauson-Khand reactions with an aldehyde or ketone component have been shown to afford synthetically versatile y-butyrolactones. Buchwald [50] and Crowe [51] independently showed that aliphatic enones and enals react with CO under Cp2Ti(PMe3)2 mediation (Scheme 11). CO insertion and thermal (or oxidative) decomposition gave diastereomerically pure bicyclic y-butyrolactones and stable Cp2Ti(CO)2. Imines did not react under the reaction conditions. 

---