The Influence of the Substrate Structure in the Tellurocyclofunctionalization Reaction of γ , δ-Unsaturated Carboxylic Acids and their Corresponding Benzyl Esters *

Áci dos car bo xí li cos γ,δ-in sa tu ra dos con ten do li ga ções du plas mo no subs ti tu í das re a gem com tri clo re tos de aril te lú rio, for ne cen do as te lu ro lac to nas es pe ra das; re a ção dos és te res ben zí li cos cor res pon den tes for ne ce o pro du to de adi ção dos tri clo re tos de aril te lú rio à li ga ção du pla. Re a ção de áci dos car bo xí li cos γ,δ-in sa tu ra dos con ten do li ga ções du plas 1,1-disubstituidas, leva a uma mis tu ra das te lu ro lac to nas es pe ra das e o pro du to de adi ção de áci do clo rí dri co à li ga ção du pla; os és te res ben zí li cos cor res pon den tes for ne cem te lu ro lac to nas como úni co pro du to. A es te re o se le ti vi da de da re a ção é ba i xa; for mam-se mis tu ras das duas pos sí ve is lac to nas di as te re o mé ri cas em re la ção apro xi ma da men te 1:1.


Introduction
The tel lu rocy clo func ti o na li za ti on was dis co ve red at the same time as the se le nocy clo func ti o na li za ti on 1 .Sin ce then the se le nocy clo func ti o na li za ti on be ca me a va lu a ble synthe tic too l 2 ; ho we ver its tel lu ri um coun ter part has re ce ived re la ti vely lit tle at ten ti on.Some time ago we re por ted a syste ma tic study on the lac to ni za ti on of γ,δ-mo no subs ti tuted acy clic and cyclic car boxy lic acids with p-met hoxypheny ltel lu ri um trich lo ri de 3 .The de tel lu ra ti on of the tel lu ro lac to nes was achi e ved by Bu 3 SnH re duc ti on 4 .The se are the only re ports on the lac to ni za ti on of un sa tu ra ted acids with ary ltel lu ri um trich lo ri des.As se ve ral ot her aspects of this re ac ti on were not yet in ves ti ga ted our group ini ti a ted a study to de ter mi ne the sco pe and li mi ta ti ons of the tel lu ro lac to ni za ti on re ac ti on.
In this work we stu di ed the in flu en ce of the tel lu ri um elec trop hi le and the ole fin struc tu re in the cour se of the tel -lu ro lac to ni za ti on of γ,δ-un sa tu ra ted car boxy lic acids exten ding the in ves ti ga ti on to the ir benzyl es ters.As the re acti on le ads to mix tu res of di as te re o me ric lac to nes, the ques ti on of the ste re o se lec vity of this pro cess was ad dressed.
The γ,δ-un sa tu ra ted car boxy lic acids 1 and the cor res poncorresponding benzyl es ter 2 5 used are show in Sche me 1.
In all ca ses, the ary ltel lu ri um trich lo ri de 4 re ac ted fas ter with 1 and 2 than the ary ltel lu ri um trich lo ri de 3 (Ta ble 1).Pro bably this dif fe ren ce in re ac ti vity is as so ci a ted to the hig her so lu bi lity of 4 in chlo ro form and the lo wer elec tron den sity on the tel lu ri um atom due to the con ju ga ti on of the oxy gen lone elec tron pair with the se cond phenyl group.The struc tu re of the subs tra te has a de ci si ve in flu en ce in the cour se of the re ac ti on and in the na tu re of the pro ducts for med.Car boxy lic acid 1a re ac ted with ary ltel lu ri um trich lo ri de 3 and 4 le a ding to the tel lu ro lac to ne 5a,b in good yi elds.The cor res pon ding benzyl es ter 2a did not give lacto nes 5a,b .The pro ducts were the car bon-carbon dou ble bond ad ducts with the ary ltel lu ri um trich lo ri des ( 6) (Scheme 2).
This re sult can be ra ti o na li zed in terms of a lo wer nu cleop hi li city of the car boxy lic oxy gen of 2a .Pro bably the first Fi gu re 1.
Ta ble 1. Re ac ti on of ary ltel lu ri um trich lo ri des with γ,δ-un sa tu ra ted car boxy lic acids and benzyl es ters.step con sists on the for ma ti on of a π com plex bet we en the dou ble bond and the elec trop hi le.In the ab sen ce of a good in ter nal nu cle op hi le, the chlo ri de ion ori gi na ted from the ary ltel lu ri um trich lo ri de at tacks the com plex le a ding to the ad duct 6. Re ac ti on of 3 and 4 with acid 1c gave the ex pec ted lacto ne 5e,f ac com pa ni ed by the car bon-carbon dou ble bond ad duct with hydroch lo ric acid (7) (Eq.1).

Subs tra te
The hydroch lo ric acid li be ra ted du ring the lac to ne forma ti on at tacks the car bon-carbon dou ble bond le a ding to a ter ti ary car be ni um ion which re acts with the chlo ri de ion gi ving 7.In the pre ce e ding case (Sche me 2) this side re ac tion was not ob ser ved in view of the lo wer sta bi lity of the car be ni um ion, which is not for med.This pro blem could not be cir cun ven ted by ad ding an ami ne to the re ac ti on me - di um to trap the HCl for med sin ce ary ltel lu ri um trich lo rides re act with ami nes 6 .
On the con trary, re ac ti on of 3 and 4 with the benzyl ester 2c gave only the tel lu ro lac to nes 5e,f in good yi elds, since benzyl chlo ri de ins te ad of hydro gen chlo ri de was for med as by-product.Si mi lar re sults were ob ta i ned with acid 1d and es ter 2d (Ta ble 1).
Con cer ning the ste re o che mi cal cour se of the re ac ti on it was ob ser ved a low ste re o se lec ti vity.Acids 1a and 1b gave the tel lu ro lac to ne in a 3:1 cis /trans ra tio.Acids 1c and 1d and es ters 2c and 2d gave the tel lu ro lac to nes in an ap pro xima tely 1:1 cis /trans ra tio (Sche me 3).The elec trop hi le had no in flu en ce in the iso me ric ra tio.
The ra ti os were de ter mi ned by NOE ex pe ri ments with the mix tu re of iso mers and by the re la ti ve in te gral of the methyl groups sig nals in the 1 H-NMR spec tra of the cru de mix tu re.In one case (lac to ne 5e ) one iso mer was se pa ra ted by suc ces si ve recrysta li za ti ons and its struc tu re was de termi ned by X-ray analy sis 7 .Com pa ri son of the NMR spectrum of the pure iso mer 5e with the mix tu re con fir med the ste re o che mistry as sig ned by 1 H-NMR analy sis.
In con clu si on, the tel lu ro lac to ni za ti on of γ,δ-un sa tu rated acids con ta i ning mo no subs ti tu ted ole fins is suc cess ful, the tel lu ro lac to ni za ti on of γ,δ-un sa tu ra ted acids con ta i ning di subs ti tu ted ole fins is not sa tis fac tory le a ding to a mix tu re of cycli za ti on pro ducts and HCl ad di ti on pro ducts to the car bon-carbon dou ble bond.Ho we ver, tel lu ro lac to ni za ti on of benzyl es ters de ri ved from the se last acids give good yields of the tel lu ro lac to nes whe re as the benzyl es ters de rived from the for mer acids give mix tu re of pro ducts, pre do mi na ting the car bon-carbon dou ble bond ad duct with the ary ltel lu ri um trich lo ri des.

Ge ne ral
Sol vents were pu ri fi ed ac cor ding to the li te ra tu re 8 .Column chro ma to graphy (flash) was per for med with 230-400 mesh, 60 Å (Merck) si li ca gel.TLC was per for med on HF-254 (Merck) pla tes, vi su a li zing with a 254 nm UV lamp and with I 2 va por or with an aci dic so lu ti on of va ni lin.Pro ton Nu cle ar Mag ne tic Re so nan ce spec tra ( 1 H-NMR) were re cor ded on a Bruc ker AC 200 ins tru ment.The chemi cal shifts are re por ted on the δ sca le (ppm) down fi eld from te tra methylsi la ne.Car bon Nu cle ar Mag ne tic Re sonan ce spec tra ( 13 C-NMR) were ob ta i ned at 50 MHz on a Bruc ker AC 200 spec tro me ter and are re por ted (ppm) re lati ve to the cen tre line of a tri plet at 77.00 ppm for CDC l 3 .Infra red (IR) spec tra were me a su red with a Per kin Elmer 1750-FT spec tro me ter.Ele men tal analy sis were per for med on a Per kin Elmer 2400 equip ment.All analy sis were perfor med by Cen tral Ana lí ti ca -Insti tu to de Qu í mi ca -USP.
Tel lu ri um te trach lo ri de 1d , p-methoxyphenyltellurium trich lo ri de 1d , p-phenoxyphenyltellurium trich lo ri de 1d , were ob ta i ned by li te ra tu re pro ce du res.
Re ac ti on of γ,δ-un sa tu ra ted car boxy lic acids (1) with ary ltel lu ri um trich lo ri des (3 or 4) A mix tu re of the ary ltel lu ri um trich lo ri de (3 or 4) (1.1 mmol) and the γ,δ-un sa tu ra ted car boxy lic acid (1) (1.0 mmol) in dry chlo ro form (15 mL) was re flu xed un til the con sump ti on of the star ting car boxy lic acid.The re ac ti on was mo ni to red by TLC.Then the sol vent was eva po ra ted and the re si due was fil te red through a si li ca gel co lumn eluting with chlo ro form.The so lu ti on was dri ed over mag nesi um sul fa te, the sol vent was eva po ra ted and the pro duct was recrysta li zed from chlo ro form / pe tro le um et her to give the cru de oil with the yi elds gi ven in Ta ble 1.When the hydroch lo ric acid aduct was for med it was se pa ra ted by column chro ma to graphy elu ting with a mix tu re of he xa ne / ethyl ace ta te (4:1) and then with chlo ro form to re mo ve the tel lu ro lac to ne.
Re ac ti on of γ,δ-un sa tu ra ted benzyl es ters (2) with ary ltellu ri um trich lo ri des (3 or 4)   The same pro ce du re des cri bed abo ve for the re ac ti on was used.The re si due ob ta i ned af ter eva po ra ti on of the chlo ro form was chro ma to grap hed on si li ca gel elu ting first with he xa ne / ethyl ace ta te (8:1) to re mo ve the benzyl chlori de and the non re ac ted benzyl es ter and then with chlo roform to re mo ve the tel lu ra ted pro ducts ( 5 or 6).

Ta ble 1 .a
(cont.) Subs tra te Elec trop hi le Pro duct a Re ac ti on time (h) Yi eld b (Mixture of the two possible diastereomers; b Yield of the recrystalized products; c Obtained as oils.No satisfactory elemental analysis were obtained.