Communications to the Editor Bull. Korean Chem. Soc. 1998, Vol. 19, No. 1 35 T T Ovex 199A A/ 259 pind rpfprpncpQ fhprpin
j .u\yig^ ^yibeim^ u, uj, jJ^a丄丄ure丄erencebI丄丄ere丄丄丄・
4. Forrecent reviews on the reactions with samarium diiodide, see: (a)Molander, G. A.; Harris, C.R. Chem.
Rev. 1996, 96, 307.(b) Molander, G. A. Org. React.
1994, 46, 211. (c) Imamoto, T.Lanthanides in Organic Synthesis;Academic Press: London, 1994;pp. 23-62. (d) Molander,G. A.; Harris, C. R.Chem.Rev. 1992, 92, 29.
5. Forreviews onthe cleavage of isoxazoline N-O bonds,
see: Curran, D. P. In AdvancesinCycloaddition;Curran, D. P. Ed.;JAI Press Inc.: London, U. K., 1988;vol 1, pp 129-189.
6. For thepreparation ofa solution of samarium diiodide in tetrahydrofuran, see: Imamoto,T.LanthanidesinOr ganic Synthesis;Academic Press:London, 1994;p21.
7. Further work is in progress to locatetheoriginoflower yield in this case and to improvetheyield of 3e.
Diastereoselective Routes in the Paterno-Buchi Reaction of Cyclic Enol Ortho Ester with Aldehydes
'Seung-Ki Park**, Seung-Joo Lee, Kwangwoo Baek, and Chan-Mo Yu*
DepartmentofChemistry, Sung Kyun Kwan University, Suwon 440-746, Korea
^Department of Chemistry, The University of Suwon, Suwon445-743, Korea Received October 20, 1997
In confluence with thewide spreadadvances in synthetic methods, particularly diastereoselectiveroutes, thePaterno- Buchi reaction hasprosperedandled to significantdevelop ments in the area of synthetic organic chemistry. 1 The Paterno-Buchireactionrepresentedbythe[2+2]photocyclo additionofcarbonyl functionality to alkeneprovides a unique way of routeto functionalizedoxetane.2 There have been several investigations directed towardsdiastereoselectivity inthis reaction using enol ethers mainlydue to the genera
tionof up to three new stereogenic centers during the pro- cess.3We recently reported the diastereoselective synthesis of highly functionalized aldolproducts of glutarates 3from the reaction of dihydropyran 1with aldehydes promoted by a Lewis acid catalyst.4 The efficiency ofthis transformation concerningdiastereocontrolledprocess has encouragedus to apply the extension of this substrateto structurally unique systems.We report herein ourdiscovery of a broadlyuseful methodforassemblingproducts 4 from the photochemical reaction of 2,2-diethoxy-3,4-dihydro-2H-pyran(2) withal
dehydes as depicted in Scheme 1. The methoddescribed hereinissuccessful withseveralaldehydesandaffords prod
uctswithuseful levels of diastereoselectivity.
Startingmaterial2was prepared in quantityby a two step sequence,purified bydistillation,and stableto storage.Treat mentof chloroacetaldehyde diethylacetalwitht-BuOK in t- BuOH affordedketene acetal 5in78% yield after distilla-
Scheme 1. (a) t-BuOK, t-BuOH, (b) 2,6-Di-tert-butylphenol (0.5 mol%), 180 °C, 2 h, sealed tube.
'Dedicated to Prof. Sang Chul Shim on the occasion of his 60 th birthday.
tion. The heteroDiels-Alder reactionof 5 withacrolein in the presence of2,6-di-tert-butylphenol(0.5 mol %)at 180
°Cfor 2h ina sealedtube affordedadduct 7 in 74% yield.5 Thestagewas thus setforthe photochemicalcycloaddition of 2 with aldehydes.
The initial study for orienting experimentsfocusedonthe feasibility of substrate 2 for the photochemical cyclization with achiralaldehydes. Preliminarystudies onthe reaction of 2 with benzaldehydeindicated that the conversion to the corresponding product couldberealizedbyirradiationof the reaction mixture at300nm. After surveying numerous con
ditions, severalkey findings emerged: (i)optimal chemical yieldsanddiastereo-selectivitieswereobserved with theuse of CH2Cl2 asa solvent compared to other solvents such as benzene and acetonitrile; (ii) a 2 : 1 mixture of aldehyde/
dihydropyran 2 provedto be the most effective in termsof chemicalyield; (iii)benzaldehyde exhibited better reactivity than aliphatic aldehydes; (iv) interesting observation was madethat theformation of rearrangedproduct9 duringthe photocycloaddition was detected. The photochemical cycloaddtionwasperformedaccording to thefollowing gen
eral procedure:A quartz tube was charged with benzalde hydeEq. (2)and dihydropyran 2 Eq. (1) dissolvedindegased CH2Cl2. The sample was irradiated at 300 nm atroomtem peraturefor3 h, and then the irradiationwas stopped. After cooledto 0°C, thereaction mixture was treated with0.5 N aqueousHCl followed byworkupandsilicagelchromatog raphy toafford inseparable oxetanes 7a and8a in a ratio of 92 : 8 asjudgedby1H NMR along with theminorproduct 9a (3 : 2diastereomeric mixture). The resultsobtained withother aldehydes aresummarized in Scheme2.
Although the exact mechanistic aspects of this transfor
mation havenot been rigorously elucidated, the following pathway could be a probablechemicalandstereochemical routes onthe basis of productpopulation. According to the mechanisticstudiesfor photocycloadd-itions,6 a putative 1,4- diradicalintermediate for theoxetane formation resulted from the additionofn,n* triplet stateof carbonyl compound to
36 Bull. Korean Chem. Soc. 1998, Vol. 19, No. 1 Communications to the Editor
i. rcho 一 300 nm, C改C0 ii. HCI
7 8 9
O
Scheme2.
a: R = Ph 57% (92:8) 8%
b: R =CH2CH2Ph 38% (88:13) 5%
c: R = C6H13 31% (91:9) <5%
Figure 1. Plausible Reaction Pathways
alkene.7 The diradical led to the formation of oxetane stereoselectivelythroughmore stableradicalorientation due to the steric and stereoelectronic factors. Therefore, treat ment ofdihydro-pyran2 with the excited benzaldehyde ini tiated by irradiation resulted inthe formation ofdiradical species as depicted in Figure 1. The observed preferencefor the relativegeometry of diradical species 10 rather than 11 could berationalized in terms of two possible transforma
tions. Cyclization of 10 ledto the formation ofsomewhat unstable 12,8 andsubsequent acidic work up provided the major oxetane 7. The formation of minorproduct9 couldbe explained bythe cyclization of thereorganized radicalspe
cies13 mainlyfrom theunstable 11. Sincetwodiastereomers 7 and 8mustbederived from the structurally rigid 12and its epimer,itfollows that 7adiffer from 8a only intherelative stereochemistry at C1. The stereochemistry of themajorcom ponent 7 in each case was determined based onthe vicinal couplingpatterns inthe1HNMRspectra. The trans-cis ste
reochemical assignments inoxetane are often based onthe magnitude of vicinal coupling constantsresulting from the optimalconformation.9 The smallercouplingconstantforHa (doublet, JHaHb=6.21 Hz at 8 5.88) obtained fromthe spec
trum of 7a (R=Ph) clearly indicated transconfiguration whereas adoublet coupling of7.88 Hz at 8 6.12 of minor component8aimplied it tobe cis. Thesemechanisticspecu lations and 1H NMRstudies clearly indicatesthatthe major components of thesetransformationshavetransstereochemi
cal relationships. These stereochemical outcomes are well consistent withpreviously reportedresults.1,10
In summary, this paper describes diastereoselectivesyn thesis of highly functionalized oxetanes by means of photocycloaddi-tion of dihydropyran 2 and achiralaldehydes in avery general way which promisesto be widelyappli cable. Further studies including synthetic application and extension of this method into enantiomeric pathways arein progress.
Acknowledgment. Generous financialsupport of this research bygrants from theMinistry of Education(BSRI 97
3420) andthe Korea Science and EngineeringFoundation (KOSEF 97-0501-02-01-3) isgratefully acknowledged.
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