D scheme

To overcome these problems with the first generation Brmsted acid-assisted chiral Lewis acid 7, Yamamoto and coworkers developed in 1996 a second-generation catalyst 8 containing the 3,5-bis-(trifluoromethyl)phenylboronic acid moiety [10b,d] (Scheme 1.15, 1.16, Table 1.4, 1.5). The catalyst was prepared from a chiral triol containing a chiral binaphthol moiety and 3,5-bis-(trifluoromethyl)phenylboronic acid, with removal of water. This is a practical Diels-Alder catalyst, effective in catalyzing the reaction not only of a-substituted a,/ -unsaturated aldehydes, but also of a-unsubstituted a,/ -unsaturated aldehydes. In each reaction, the adducts were formed in high yields and with excellent enantioselectivity. It also promotes the reaction with less reactive dienophiles such as crotonaldehyde. Less reactive dienes such as isoprene and cyclohexadiene can, moreover, also be successfully employed in reactions with bromoacrolein, methacrolein, and acrolein dienophiles. The chiral ligand was readily recovered (>90%). 

Pyridones can also be converted to 2-chloropyridines by exchanging the carbonyl functionality using phosphoroxychloride (POCI3) [72]. A combination of N-halosuccinimides and triphenylphosphine has also been applied to introduce halogens in this position [73]. The carbonyl functionality in 2-pyridones makes these systems reactive towards nucleophiles as well, which add in 1,4-reactions with displacement of halides [74]. The use of transition metal mediated couplings like Heck, and Suzuki have also been successfully applied on halogenated 2-pyridones (d. Scheme 10) [36,75]. 

More general processes rely on variations of the Guareschi-Thorpe reaction [14] where condensations between 1,3-dicarbonyls and cyanoacetamide yield functionalized monocyclic 2(lff)-pyridones (a and b. Scheme 2) [15, 16]. Unless the carbonyls are sufficiently different in reactivity, the reaction suffers from poor regioselectivity. The use of 3-alkoxy or 3-amino enones instead of 1,3-dicarbonyls has proven to be a versatile and reliable synthetic methodology where the 1,4-addition controls the regioselective outcome (c and d. Scheme 2) [17-19]. 

Tridentate amido-amidinate ligands have also been constructed starting from (lR,2R)-diaminocyclohexane (cf. Section IV.D). Scheme 181 illustrates the use of such ligand in the preparation of novel amidinato-titanium alkoxide com-plexes. ... 

Nitrophenyl 2, 3 -0,0-cyclic phosphites 18a-d were formed rapidly and cleanly as two diasteroisomers in the reaction of 5 -0-protected ribonucleosides 16a-d with tris(4-nitrophenyl) phosphite (17) in the presence of pyridine [reaction time less than 3 min at room temperature in DMF/pyridine (9 1 v/v) solution (monitored by 31P-NMR )]. Their sulfhydrolysis, which is also very rapid using an excess of hydrogen sulfide at room temperature, gave cyclic //-phosphonothioatcs 19a-d (Scheme 7) [22], In this reaction, the corresponding 2, 3 -0,0-cyclic... 

Similarly, hydroboration of p-methoxystyrene with HBcat using zwitterionic [(P-P)Rh(//6-catBcat)] ((4), P-P = dppe, dippe) or [(dippe)Rh(r/3-2-IVIc-a 11 y 1)] (dippe= l,2-bis(diisopropylphosphine)ethane) gave 99% of the internal hydroboration product. Complications arose in studying reactions of hindered styrenes due to the appearance of not only the hydroboration products (A and B), but also the dehydrogenative borylation products C and D (Scheme 5).32 The ratio of products was found to be highly catalyst dependent (Table 2).33... 

Compound 56 is the first branch point intermediate in the analog syntheses, furnishing 90 upon oxidative cyclization with MnC>2 and deprotection with Mgl2. Intermediate 56 was also benzylated to protect the C5,C5 -naphthols in preparation for ester hydrolysis, which provided the next key branch point intermediate, bisacid 91. Ester hydrolysis here with aqueous base was surprising facile relative to intermediate 62, en route to (+)-calphostin D (Scheme 7.14). Presumably, the smaller C7,C7 -groups alleviate the steric gearing that hinders the reactivity of the C3,C3 -esters. 

Fig. 5.22 (A-D) Scheme of Au nanoparticle growth between bilayers (A) nucleation at bilayer interfaces and bulk-like growth when particle size is smaller than the lamellar d spacing (B) once the transversal particle size is larger than the d spacing, growth is slowed by the bilayers, the transversal Au3+flux being limited, leadingto elongated particles shapes (C) when the constraint exerted by the particles on the bilayers is...

The jt-electron density distribution in nitronates can be described by borderline resonance structures A-D (Scheme 3.84). 

The pioneering study (337) was performed more than 25 years ago and, unfortunately, the principal results of this study were not published in reviewed journals. Six-membered cyclic nitronates (249a-g) and dipolarophiles (250a-d) (Scheme 3.168) were investigated. This appropriate choice of compounds made it possible to eliminate problems associated with stereoisomerism of nitronates. 

Boger and coworkers [51] have also extended their very useful pyrrole methodology to the preparation of ningalin D (Scheme 20). A tetrasubsti-tuted pyrrole (100) is prepared in the usual manner (see Scheme 3) via Diels-Alder/retrograde Diels-Alder chemistry involving azines that undergo... 

Nitrosonium complexes 20a-d of l-R-2-methylacenaphthylenes 21a-d (Scheme 15) can be considered as complexes with two-electron ligands, as unlike complexes of other polycyclic aromatic compounds (8, 52), nitrosonium... 

Table 15.4 Preparation of sialodendrimer-chitosan hybrid 30a-d (Scheme 12)fl...

Quantum-chemical calculations furnished very similar energies of 1,2-cyclobuta-diene (13) and the diradical 13-D (Scheme 6.5), with a small preference for the latter. Since the ring opening of 13 or 13-D was estimated to be highly exothermic (74.5 kcal mol-1) because of the inherent strain, the barrier for this step must be low [24]. 

The mismatched R/S pairing could lead to the anti,syn adduct through transition state C and the syn,anti adduct via D (Scheme 9.30). The former pathway entails non-Felkin-Anh addition but anti disposed methyl and aldehyde substituents. Transition state D proceeds through the Felkin-Anh mode of carbonyl addition but requires eclipsing of the methyl and aldehyde substituents. This interaction is the more costly one and thus disfavors the syn,anti adduct. 

Fig. 6a-d. Scheme of bead-jump moves for a linear chain on a simple cubic lattice a bent (end move) b bent (inner move) c crankshaft (end move) d crankshaft (inner move). Solid lines Initial bonds broken lines final bonds (alternative possibilities included)... 

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