<script src="//code.jquery.com/jquery-latest.min.js"></script>
<div class="block1">

<div class="block2_1">2_1 раз</div>
<div class="block2_2">
    <div class="block1">
        <div class="block2_1">2_1 два</div>
        <div class="block2_2">
            <div class="block1">
                <div class="block2_1">2_1 три</div>
                <div class="block2_2"></div>
            </div>
        </div>
    </div>
</div>

</div>

<script>
$(function(){
  $('.block1').each(function() {
    var $data = $(this).clone();
    $data.find('.block1').remove();
    var data1 = $data.find('.block2_1').html();
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    alert(data1);
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  $('.block1').replaceWith('<div>replaced</div>');
});
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