<MultiModal>![O=C1C(CC(Br)=C)CCCC1(C=C)O>$Bu_3SnH$, $AIBN$>A, where $Bu_3SnH$ is tributyltin hydride, $AIBN$ is azobisisobutyronitrile, and the hydroxyl and hydrogen groups on the two α-positions of the carbonyl group are cis](/media/uploads/7b4deec3-e7fd-4411-b7bd-6d4abc5af9ea.png)</MultiModal>

Please select the major product A of the reaction.
<MultiModal>![O=C1C(CC(Br)=C)CCCC1(C=C)O>$Bu_3SnH$, $AIBN$>A，其中$Bu_3SnH$为三丁基氢化锡，$AIBN$为偶氮二异丁腈，羰基的两个α-位上的羟基与氢为顺式](/media/uploads/7b4deec3-e7fd-4411-b7bd-6d4abc5af9ea.png)</MultiModal>

请选择反应的主产物A
<MultiModal>![O=C1C(C2)CCCC1(O)CCC2=C](/media/uploads/e26cd475-3d9a-47d2-8709-c826c7715869.png)</MultiModal>
<MultiModal>![O=C1C(C2)CCCC(O)C1CC2=C](/media/uploads/dc66f7df-df69-4014-afee-ff28a9bf795d.png)</MultiModal>
<MultiModal>![O=C1C(CC=C)CCCC1(C=C)O](/media/uploads/059c5baf-a4c7-4a9c-ba9a-fc8e28854033.png)</MultiModal>
<MultiModal>![O=C1C(CC2=C)CCCC1(C2C)O](/media/uploads/7d76507e-310a-4dbc-a10a-540e26bfbbc3.png)</MultiModal>
<MultiModal>![O=C1C(C2)CCCC1(O)CCC2=C](/media/uploads/e26cd475-3d9a-47d2-8709-c826c7715869.png)</MultiModal>
<MultiModal>![O=C1C(C2)CCCC(O)C1CC2=C](/media/uploads/dc66f7df-df69-4014-afee-ff28a9bf795d.png)</MultiModal>
<MultiModal>![O=C1C(CC=C)CCCC1(C=C)O](/media/uploads/059c5baf-a4c7-4a9c-ba9a-fc8e28854033.png)</MultiModal>
<MultiModal>![O=C1C(CC2=C)CCCC1(C2C)O](/media/uploads/7d76507e-310a-4dbc-a10a-540e26bfbbc3.png)</MultiModal>
After radical initiation, an alkenyl radical is first generated.

<MultiModal>![O=C1C(C[C]=C)CCCC1(C=C)O](/media/uploads/7496fdcd-dff3-45ee-88c7-ada7bc1ea3e2.png)</MultiModal>

<Checkpoint>[After radical initiation, an alkenyl radical O=C1C(C[C]=C)CCCC1(C=C)O is first generated](1)</Checkpoint>

Since a secondary carbon radical is more stable than a primary carbon radical, a 7-endo-trig radical cyclization then occurs, forming a secondary carbon radical.

<MultiModal>![O=C1C(C2)CCCC1(O)[C]CC2=C](/media/uploads/e38777c5-a4f9-4085-b353-3eb763573738.png)</MultiModal>

<Checkpoint>[Since a secondary carbon radical is more stable than a primary carbon radical, a 7-endo-trig radical cyclization then occurs, forming a secondary carbon radical O=C1C(C2)CCCC1(O)[C]CC2=C](1)</Checkpoint>

This radical then attacks the carbon atom of the carbonyl group, rapidly forming an oxygen radical containing a three-membered ring.

<MultiModal>![[O]C12C(C3)CCCC1(O)C2CC3=C](/media/uploads/e60ca567-2a35-4d6c-b9a2-15be49d726e7.png)</MultiModal>

<Checkpoint>[This radical then attacks the carbon atom of the carbonyl group, rapidly forming an oxygen radical containing a three-membered ring [O]C12C(C3)CCCC1(O)C2CC3=C](1)</Checkpoint>

The three-membered ring then undergoes cleavage, forming a more stable radical intermediate.

<MultiModal>![O=C1C(C2)CCC[C](O)C1CC2=C](/media/uploads/11ff5723-7819-42f0-8b74-6c1c102eda8c.png)</MultiModal>

<Checkpoint>[The three-membered ring then undergoes cleavage, forming a more stable radical intermediate O=C1C(C2)CCC[C](O)C1CC2=C](1)</Checkpoint>

Finally, this radical is quenched by $Bu_3SnH$ to obtain the final product A.

<Checkpoint>[Finally, this radical is quenched by $Bu_3SnH$](1)</Checkpoint>
自由基引发之后首先生成烯基自由基

<MultiModal>![O=C1C(C[C]=C)CCCC1(C=C)O](/media/uploads/7496fdcd-dff3-45ee-88c7-ada7bc1ea3e2.png)</MultiModal>

<Checkpoint>[自由基引发之后首先生成烯基自由基O=C1C(C[C]=C)CCCC1(C=C)O](1)</Checkpoint>

由于二级碳自由基比一级碳自由基更稳定，随后发生7-endo-trig自由基成环，形成二级碳自由基

<MultiModal>![O=C1C(C2)CCCC1(O)[C]CC2=C](/media/uploads/e38777c5-a4f9-4085-b353-3eb763573738.png)</MultiModal>

<Checkpoint>[二级碳自由基比一级碳自由基更稳定，随后发生7-endo-trig自由基成环，形成二级碳自由基O=C1C(C2)CCCC1(O)[C]CC2=C](1)</Checkpoint>

该自由基随即进攻羰基的碳原子快速形成含三元环的氧自由基

<MultiModal>![[O]C12C(C3)CCCC1(O)C2CC3=C](/media/uploads/e60ca567-2a35-4d6c-b9a2-15be49d726e7.png)</MultiModal>

<Checkpoint>[该自由基随即进攻羰基的碳原子快速形成含三元环的氧自由基[O]C12C(C3)CCCC1(O)C2CC3=C](1)</Checkpoint>

随后三元环发生裂解，形成更加稳定的自由基中间体

<MultiModal>![O=C1C(C2)CCC[C](O)C1CC2=C](/media/uploads/11ff5723-7819-42f0-8b74-6c1c102eda8c.png)</MultiModal>

<Checkpoint>[随后三元环发生裂解，形成更加稳定的自由基中间体O=C1C(C2)CCC[C](O)C1CC2=C](1)</Checkpoint>

最后该自由基被$Bu_3SnH$所淬灭，得到最终产物A

<Checkpoint>[最后该自由基被$Bu_3SnH$所淬灭](1)</Checkpoint>