Concept explainers
(a)
Interpretation: The formation of both the isomeric products from the starting material by a sigmatropic rearrangement involving a
Concept introduction: In the sigmatropic rearrangement, the rearrangement of pi bond and the breaking of sigma bond take place. This results in the generation of new sigma bond in the product. In this type of rearrangement, the number of pi bonds remains constant in the reactant as well as in the product.
(b)
Interpretation: The reasons as to why
Concept introduction: In the sigmatropic rearrangement, the rearrangement of pi bond and the breaking of sigma bond take place. This results in the generation of new sigma bond in the product. In this type of rearrangement, the number of pi bonds remains constant in the reactant as well as in the product.
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ORGANIC CHEMISTRY W/BIO...-STUD.SOLN.
- The bicyclic alkene P can be prepared by thermal electrocyclic ringclosure from cyclodecadiene Q or by photochemical electrocyclic ringclosure from cyclodecadiene R. Draw the structures of Q and R, andindicate the stereochemistry of the process by which each reactionoccurs.arrow_forwardGive the structure(s) of the product(s) of each reaction. Indicate any relative stereochemistry, assuming that each of the Diels-Alder reactions will proceed with endo selectivity. If more than one product is formed, please indicate which one will be the major product or if they will be formed in equal amounts. If no reaction is expected to occur under the indicated conditions, then write "no reaction" or NR, and explain why you would expect nothing to occur. In all cases, equimolar amounts of both components/reagents are present unless indicated otherwise.arrow_forwardA solution of 5-methylcyclopenta-1,3-diene rearranges at roomtemperature to a mixture containing 1-methyl-, 2-methyl-, and 5-methylcyclopenta-1,3-diene. (a) Show how both isomeric products areformed from the starting material by a sigmatropic rearrangementinvolving a C–H bond. (b) Explain why 2-methylcyclopenta-1,3-diene isnot formed directly from 5-methylcyclopenta-1,3-diene by a [1,3]rearrangement.arrow_forward
- Diels–Alder reaction of a monosubstituted diene (such as CH2=CH–CH=CHOCH3) with a monosubstituted dienophile (such as CH2=CHCHO)gives a mixture of products, but the 1,2-disubstituted product oftenpredominates. Draw the resonance hybrid for each reactant, and use thecharge distribution of the hybrids to explain why the 1,2-disubstitutedproduct is the major product.arrow_forwardWhen 1,2-dimethylcyclopentene undergoes hydroboration–oxidation, one diastereomerof the product predominates. Show why this addition is stereospecific, and predict thestereochemistry of the major product.arrow_forward(a) A compound known to be a substituted cyclohexanone derivative has lamda max of 235 nm. Could this compound be a conjugated dienone? explain (b) (i)For this compound, how many nm must be accounted for by substituents? (ii) What are the substituents and the points of substitution that may occur having accounted for the 20nm?arrow_forward
- (a) Give a mechanism for this reaction, showing how the two products arise as aconsequence of the resonance-stabilized intermediate.(b) The bromination of cyclohexene using NBS gives only one major product, as shown onthe previous page. Explain why there is no second product from an allylic shift.arrow_forwardExplain, with reference to the mechanism, why addition of one equivalent of HCl to diene A forms only two products of electrophilic addition, even though four constitutional isomers are possible.arrow_forwardStereoisomers of α-bromocinnamic acid: draw the reaction mechiansim with arows for the dehydrobromination of 2,3-dibromo-3-phenylpropanoic acid with ethanol and KOH to produce (Z)-α-bromocinnamic acid and (E)-α-bromocinnamic acid. Also draw the transition state for The (Z)-isomer syn-periplanar transition state and anti-periplanar transition state of the (E)-isomer.arrow_forward
- Give cis/trans-, E/Z-, R/S- and/or M/P-descriptor(s) for each of the following compounds:arrow_forwarda) What product is formed from the [1,7] sigmatropic rearrangement of a deuterium in the following triene? (b) Does this reaction proceed in a suprafacial or antarafacial manner under thermal conditions? (c) Does this reaction proceed in a suprafacial or antarafacial manner under photochemical conditions?arrow_forwardThe reaction of butan-2-ol with concentrated aqueous HBr goes with partial racemization, giving more inversion thanretention of configuration. Propose a mechanism that accounts for racemization with excess inversion.(b) Under the same conditions, an optically active sample of trans-2-bromocyclopentanol reacts with concentrated aqueous HBr to give an optically inactive product, (racemic) trans-1,2-dibromocyclopentane. Proposea mechanism to show how this reaction goes with apparently complete retention of configuration, yet withracemization. (Hint: Draw out the mechanism of the reaction of cyclopentene with Br2 in water to give thestarting material, trans-2- bromocyclopentanol. Consider how parts of this mechanism might be involved in thereaction with HBr.)arrow_forward