The preparation of substituted 3,4-dihydro-l(2H)-naphthalenones from benzocyclobutenones via sequential thermal electrocyclic reactions

Pd-Catalyzed tandem C–C/C–O/C–H single bond cleavage of 3-allyloxybenzocyclobutenols

A Pd-catalyzed skeletal rearrangement of 3-allyloxybenzocyclobutenols was achieved, which involved tandem C–C/C–O bond cleavage and C–H allylic substitution.

Regiospecific Synthesis of Carboxylated and Simple α-Tetralones with Homophthalates and Various Acrylates by a Simple Condensation Method

Various carboxylated enols (3) have been prepared by taking advantage of simple condensation reactions of various homophthalates (1) with different acrylates (2), and these enols under heating with two different solvents (DMF and DMSO), afforded ten specific α-tetralones in good to moderate yields using NaCl as a catalyst.

Construction of tetralin skeletons based on rhodium-catalysed site-selective ring opening of benzocyclobutenols

A rhodium complex induces site-selective alkene insertion into the C(sp²)–C(sp³) bond of benzocyclobutenols to furnish tetralins diastereoselectively.

Concise Synthesis of Functionalized Benzocyclobutenones

A concise approach to access functionalized benzocyclobutenones from 3-halophenol derivatives is described. This modified synthesis employs a [2+2] cycloaddition between benzynes generated from dehydrohalogenation of aryl halides using LiTMP and acetaldehyde enolate generated from n-BuLi and THF, followed by oxidation of the benzocyclobutenol intermediates to provide benzocyclobutenones. The [2+2] reaction can be run on a 10-gram scale with an increased yield. A number of functional groups including alkenes and alkynes are tolerated. Coupling of benzynes with ketene silyl acetals to give 8-substituted benzocyclobutenones is also demonstrated.

A convenient preparation of functionalized 1,8-dioxygenated naphthalenes from 6-alkoxybenzocyclobutenones

An alternate route for the synthesis of naphthalene building blocks 1-4 has been developed, starting from readily available 6-alkoxybenzocyclobutenones. As thermolysis of a propynylbenzocyclobutenol only provided the naphthalene in low yield, allenyl-substituted benzocyclobutenols were investigated. The desired allenic precursors were prepared by a two-step procedure, which involved hydroxyl-directed reduction of the chloropropargylbenzocyclobutenols obtained from addition of lithiopropargyl chloride to the benzocyclobutenones. Thermolysis of the allenic alcohols gave the desired naphthalenes in good yields.

Stereoselective Synthesis of (Z)- and (E)-Allylic Silanes by Copper-Mediated Substitution Reactions of Allylic Carbamates with Grignard Reagents

Both (Z)- and (E)-allylic silanes were prepared with high stereoselectivity by the copper-mediated substitution of allylic carbamates by organometallic reagents. The reaction of alkylmagnesium reagents with (E)-allylic carbamates provides (Z)-allylic silanes, whereas both alkylmagnesium and alkyllithium reagents react with (Z)-allylic carbamates to afford (E)-allylic silanes. Because Grignard reagents are often more facile to prepare than alkyllithium species, these reagents are the optimal nucleophiles for the synthesis of both (Z)- and (E)-allylic silanes. This method also allows readily available nonracemic allylic carbamates to be converted to chiral, nonracemic (Z)- and (E)-allylic silanes with high stereoselectivity.

Intramolecular Cyclizations of o-Acylbenzyllithiums. Formation of Benzocyclobuten-1-ol Derivatives and Their Thermal Isomerization

The formation of benzocyclobutenol derivatives by intramolecular cyclizations of o-acylbenzyllithiums is described. Treatment of o-(trialkylsilylmethyl)phenyl ketones with lithium diisopropylamide (LDA) followed by quenching of the resulting benzylic carbanions with chlorotrialkylsilane resulted in stereoselective formation of the corresponding 1-trialkylsiloxy-2-(trialkylsilyl)benzocyclobutenes in good yields. Subsequently, o-acyl-m-methoxybenzyllithiums were found to work well in cyclization to benzocyclobuten-1-ol derivatives. The reaction of 2-benzoyl-3,4,5-trimethoxybenzyllithium, generated in situ by deprotonation of 6-methyl-2,3,4-trimethoxybenzophenone with LDA, with chlorotrimethylsilane afforded the corresponding 1-(trimethylsiloxy)benzocyclobutene. Cyclization of 2-pivaloyl-3-methoxybenzyllithiums, generated in situ from tert-butyl 2-methyl-6-methoxyphenyl ketones upon deprotonation with LDA, proceeded spontaneously even at -78 degrees C to give the corresponding benzocyclobuten-1-ols. We also describe the results of thermal isomerization of these 1-trimethylsiloxy-2-(trialkylsilyl)benzocyclobutenes.