Boronic ring closure with

Fenoldopam (76) is an antihypertensive renal vasodilator apparently operating through the dopamine system. It is conceptually similar to trepipam. Fenoldopam is superior to dopamine itself because of its oral activity and selectivity for dopamine D-1 receptors (D-2 receptors are as.sociated with emesis). It is synthesized by reduction of 3,4-dimethoxyphenylacetonitrile (70) to dimethoxyphenethylamine (71). Attack of diis last on 4-methoxystyrene oxide (72) leads to the product of attack on the epoxide on the less hindered side (73). Ring closure with strong acid leads to substituted benzazepine 74. O-Dealkylation is accomplished with boron tribromide and the catechol moiety is oxidized to the ortho-quinone 75. Treatment with 9NHC1 results in conjugate (1,6) chloride addition and the formation of fenoldopam (76) [20,21]. 

The total syntheses of (+)-secosyrins 1 and 2 was achieved and their relative and absolute stereochemistry was unambiguously established by C. Mukai and co-workers. To construct the spiro skeleton of these natural products, the intramolecular Nicholas reaction was utilized. The alkyne substrate was first converted to the dicobalt hexacarbonyl complex by treatment with Co2(CO)8 in ether. Exposure of the resulting complex to boron trifluoride etherate at room temperature brought about the ring closure with inversion of configuration at C5 to afford the expected tetrahydrofuran derivative. The minor product was the C5 epimer which was formed only in 15% yield. 

An excellent example of the use of Suzuki polycondensation is the synthesis of ladder-type PPPs (67) (see Scheme 6.16) [84]. A precursor polymer 79 is prepared by AA-BB coupling and then converted to the ladder polymers by polymer analogous reactions. Reduction followed by ring closure with boron trifluoride produces a polymer (67a) with bridgehead hydrogens, while addition of methyl lithium instead of reduction leads to Me-LPPP (67b) with methyls at the bridgeheads. 

A modification of the synthesis of the tetrarylindenofluorenes 186 shown in Scheme 86 has been used to prepare ladder-type pentaphenylenes (Scheme 88) [283]. The dibromoterephthalate was coupled with two equivalents of a flu-orene boronate instead of a benzene boronic acid to give a pentaphenylene 188. Reaction with an aryl lithium, followed by ring closure with boron trifluoride, gave the ladder-type pentamers 189, which were brominated to prepare dibromomonomers that were then polymerised using nickel(O). 

Trifluoroacetic anhydride I boron fluoride Ring closure with acetals Pomeranz-Fritsch isoquinoline ring closure... 

Watanabe reports a new method for the direct conversion of o-choroacetaldehyde N,N-disubstituted hydrazones into 1-aminoindole derivatives 93 by palladium-catalyzed intramolecular ring closure of 92 in the presence of P Bu3 or the bisferrocenyl ligand 94 <00AG(E)2501>. When X = Cl, this cyclizative process can be coupled with other Pd-catalyzed processes with nucleophilic reagents (e.g., amines, azoles, aryl boronic acids) so as to furnish indole derivatives with substituents on the carbocyclic ring. 

The boronic acid ester B was synthesized by transesterification of the corresponding pinacolester A with (lR,2R)-l,2-dicyclohexyl-l,2-dihydroxyethane. Stereoselective chlorination of B was carried out with (dichloromethyl) lithium and zinc chloride. Reaction of the obtained chloroboronic ester C with lithio 1-decyne followed by oxidation of the intermediate D with alkaline hydrogen peroxide afforded the propargylic alcohol E. Treatment with acid to saponify the tert-butyl ester moiety and to achieve ring closure, produced lactone F. Finally, Lindlar-hydrogenation provided japonilure 70 in an excellent yield and high enantiomeric purity. 

Another route to ring (7) involves introduction of the boron atom last, in the reaction between boron tribromide and an aromatic tosylhydrazone as exemplified in Scheme 26. The yields in the ring closure are not as high as those mentioned above but the main advantage with this method is the ready availability of the starting materials (78HCA325). 

Ring-closing dehydration, with fission or heterocyclic ring closures using pyridine hydrochloride,202,203 or boron trichloride204 may be used for the preparation of molecules from types A and B. 

Ring closure occurs also with main group metals, e.g., with suitable compounds of magnesium, boron, etc.49 (Fig. 12). 

Aryl coupling to a benzylic site has also been observed the monophenol (67) yielded the aryltetralin (68 55%), with thallium trifluoroacetate-boron trifluoride. Probably oxidation to quinone methide precedes the ring closure. Separate oxidation and cyclization steps were employed in the synthesis of ( )-thaliphotphine acetate. ( )-Codamine (69) underwent Wessely oxidation with lead tetraacetate to the acetoxycyclohexadienone (70), which closed in acetic anhydride-acid to ( )-thaliphorphine acetate (71), albeit in modest overall yield (14%). ... 

Scherf and Mullen prepared (Scheme 47) the ladder-type polyphenylene (LPPP, 5) with methine bridges [126-129], via a poly(diacylphenylene-co-phenylene) precursor copolymer 103 obtained by an AA-BB type Suzuki polycondensation. The key step is the polymer analogous Friedel-Crafts ringclosing reaction on the polyalcohol 104, obtained by the reduction of 103. This was found to proceed quickly and smoothly upon addition of boron-trifluoride to a solution of 104 in dichloromethane. The reaction appeared to be complete by both NMR and MALDI-TOF analysis, indicating the presence of less than 1% of defects due to incomplete ring closure. LPPPs with num-... 

The hydrogen at the methine bridges can be replaced with a methyl group to give Me-LPPP (106) (Scheme 49) [132]. This is achieved by treating the precursor polymer 103 with methyl lithium, followed by ring-closure of the resulting polyalcohol 107 with boron-trilluoride as in the preparation of 5. 

In parallel, Scherf et al. [128] have also reported a ladder-type polymer containing carbazole moieties. Their strategy is quite similar to those presented above the authors copolymerized the carbazole moiety with a phenyl bearing two benzoyl side chains. After the polymerization is completed, the ketone moieties located on the phenyl are reduced into alcohol groups by a reaction with MeLi. The ring-closure reaction of the polyalcohol was carried out by treatment with boron trifluoride etherate, affording the ladder polymer. 

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